AsyncPipe; table schemata; migrations; File logging

src/lifecycle/Application.ts

@@ -15,6 +15,7 @@ import {RunLevelErrorHandler} from './RunLevelErrorHandler'
 import {Unit, UnitStatus} from './Unit'
 import * as dotenv from 'dotenv'
 import {CacheFactory} from '../support/cache/CacheFactory'
+import {FileLogger} from '../util/logging/FileLogger'
 
 /**
  * Helper function that resolves and infers environment variable values.
@@ -225,6 +226,12 @@ export class Application extends Container {
         const logging: Logging = this.make<Logging>(Logging)
 
         logging.registerLogger(standard)
+
+        if ( this.env('EXTOLLO_LOGGING_ENABLE_FILE') ) {
+            const file: FileLogger = this.make<FileLogger>(FileLogger)
+            logging.registerLogger(file)
+        }
+
         logging.verbose('Attempting to load logging level from the environment...')
 
         const envLevel = this.env('EXTOLLO_LOGGING_LEVEL')

src/migrations/2021-07-23T19:44:00.000Z_CreateSessionsTable.migration.ts

@@ -0,0 +1,39 @@
+import {Inject, Injectable} from '../di'
+import {ConstraintType, DatabaseService, FieldType, Migration, Schema} from '../orm'
+
+/**
+ * Migration that creates the sessions table used by the ORMSession backend.
+ */
+@Injectable()
+export default class CreateSessionsTableMigration extends Migration {
+    @Inject()
+    protected readonly db!: DatabaseService
+
+    async up(): Promise<void> {
+        const schema: Schema = this.db.get().schema()
+        const table = await schema.table('sessions')
+
+        table.primaryKey('session_uuid', FieldType.varchar)
+            .required()
+
+        table.column('session_data')
+            .type(FieldType.json)
+            .required()
+            .default('{}')
+
+        table.constraint('session_uuid_ck')
+            .type(ConstraintType.Check)
+            .expression('LENGTH(session_uuid) > 0')
+
+        await schema.commit(table)
+    }
+
+    async down(): Promise<void> {
+        const schema: Schema = this.db.get().schema()
+        const table = await schema.table('sessions')
+
+        table.dropIfExists()
+
+        await schema.commit(table)
+    }
+}

src/migrations/2021-07-24T10:31:00.000Z_CreateUsersTable.migration.ts

@@ -0,0 +1,47 @@
+import {Inject, Injectable} from '../di'
+import {DatabaseService, FieldType, Migration, Schema} from '../orm'
+
+/**
+ * Migration that creates the users table used by @extollo/lib.auth.
+ */
+@Injectable()
+export default class CreateUsersTableMigration extends Migration {
+    @Inject()
+    protected readonly db!: DatabaseService
+
+    async up(): Promise<void> {
+        const schema: Schema = this.db.get().schema()
+        const table = await schema.table('users')
+
+        table.primaryKey('user_id')
+            .required()
+
+        table.column('first_name')
+            .type(FieldType.varchar)
+            .required()
+
+        table.column('last_name')
+            .type(FieldType.varchar)
+            .required()
+
+        table.column('password_hash')
+            .type(FieldType.text)
+            .nullable()
+
+        table.column('username')
+            .type(FieldType.varchar)
+            .required()
+            .unique()
+
+        await schema.commit(table)
+    }
+
+    async down(): Promise<void> {
+        const schema: Schema = this.db.get().schema()
+        const table = await schema.table('users')
+
+        table.dropIfExists()
+
+        await schema.commit(table)
+    }
+}

src/orm/DatabaseService.ts

@@ -41,7 +41,7 @@ export class DatabaseService extends AppClass {
      * Get a connection instance by its name. Throws if none exists.
      * @param name
      */
-    get(name: string): Connection {
+    get(name = 'default'): Connection {
         if ( !this.has(name) ) {
             throw new ErrorWithContext(`No such connection is registered: ${name}`)
         }

src/orm/builder/AbstractBuilder.ts

@@ -1,4 +1,4 @@
-import {Inject} from '../../di'
+import {Inject, Injectable} from '../../di'
 import {DatabaseService} from '../DatabaseService'
 import {
     Constraint, ConstraintConnectionOperator,
@@ -9,7 +9,7 @@ import {
     SpecifiedField,
 } from '../types'
 import {Connection} from '../connection/Connection'
-import {deepCopy, ErrorWithContext} from '../../util'
+import {deepCopy, ErrorWithContext, Maybe} from '../../util'
 import {EscapeValue, QuerySafeValue, raw} from '../dialect/SQLDialect'
 import {ResultCollection} from './result/ResultCollection'
 import {AbstractResultIterable} from './result/AbstractResultIterable'
@@ -24,6 +24,7 @@ export type ConstraintGroupClosure<T> = (group: AbstractBuilder<T>) => any
  * A base class that facilitates building database queries using a fluent interface.
  * This can be specialized by child-classes to yield query results of the given type `T`.
  */
+@Injectable()
 export abstract class AbstractBuilder<T> extends AppClass {
     @Inject()
     protected readonly databaseService!: DatabaseService
@@ -55,6 +56,9 @@ export abstract class AbstractBuilder<T> extends AppClass {
     /** The connection on which the query should be executed. */
     protected registeredConnection?: Connection
 
+    /** Raw SQL to use instead. Overrides builder methods. */
+    protected rawSql?: string
+
     /**
      * Create a new, empty, instance of the current builder.
      */
@@ -80,6 +84,7 @@ export abstract class AbstractBuilder<T> extends AppClass {
         bldr.registeredGroupings = deepCopy(this.registeredGroupings)
         bldr.registeredOrders = deepCopy(this.registeredOrders)
         bldr.registeredConnection = this.registeredConnection
+        bldr.rawSql = this.rawSql
 
         return bldr
     }
@@ -115,6 +120,11 @@ export abstract class AbstractBuilder<T> extends AppClass {
         return deepCopy(this.registeredOrders)
     }
 
+    /** Get the raw SQL overriding the builder methods, if it exists. */
+    public get appliedRawSql(): Maybe<string> {
+        return this.rawSql
+    }
+
     /** Get the source table for this query. */
     public get querySource(): QuerySource | undefined {
         if ( this.source ) {
@@ -555,6 +565,21 @@ export abstract class AbstractBuilder<T> extends AppClass {
         return Boolean(result.rows.first())
     }
 
+    /**
+     * Set the query manually. Overrides any builder methods.
+     * @example
+     * ```ts
+     * (new Builder())
+     *  .raw('SELECT NOW() AS example_column')
+     *  .get()
+     * ```
+     * @param sql
+     */
+    raw(sql: string): this {
+        this.rawSql = sql
+        return this
+    }
+
     /**
      * Adds a constraint to this query. This is used internally by the various `where`, `whereIn`, `orWhereNot`, &c.
      * @param preop

src/orm/builder/Builder.ts

@@ -1,5 +1,5 @@
 import {ErrorWithContext} from '../../util'
-import {Container} from '../../di'
+import {Container, Injectable} from '../../di'
 import {ResultIterable} from './result/ResultIterable'
 import {QueryRow} from '../types'
 import {AbstractBuilder} from './AbstractBuilder'
@@ -8,6 +8,7 @@ import {AbstractResultIterable} from './result/AbstractResultIterable'
 /**
  * Implementation of the abstract builder class that returns simple QueryRow objects.
  */
+@Injectable()
 export class Builder extends AbstractBuilder<QueryRow> {
     public getNewInstance(): AbstractBuilder<QueryRow> {
         return Container.getContainer().make<Builder>(Builder)

src/orm/connection/Connection.ts

@@ -5,6 +5,7 @@ import {AppClass} from '../../lifecycle/AppClass'
 import {Inject, Injectable} from '../../di'
 import {EventBus} from '../../event/EventBus'
 import {QueryExecutedEvent} from './event/QueryExecutedEvent'
+import {Schema} from '../schema/Schema'
 
 /**
  * Error thrown when a connection is used before it is ready.
@@ -61,15 +62,11 @@ export abstract class Connection extends AppClass {
      */
     public abstract close(): Promise<void>
 
-    // public abstract databases(): Promise<Collection<Database>>
-
-    // public abstract database(name: string): Promise<Database | undefined>
-
-    // public abstract database_as_schema(name: string): Promise<Database>
-
-    // public abstract tables(database_name: string): Promise<Collection<Table>>
-
-    // public abstract table(database_name: string, table_name: string): Promise<Table | undefined>
+    /**
+     * Get a Schema on this connection.
+     * @param name
+     */
+    public abstract schema(name?: string): Schema
 
     /**
      * Fire a QueryExecutedEvent for the given query string.

src/orm/connection/PostgresConnection.ts

@@ -6,6 +6,8 @@ import {collect} from '../../util'
 import {SQLDialect} from '../dialect/SQLDialect'
 import {PostgreSQLDialect} from '../dialect/PostgreSQLDialect'
 import {Logging} from '../../service/Logging'
+import {Schema} from '../schema/Schema'
+import {PostgresSchema} from '../schema/PostgresSchema'
 
 /**
  * Type interface representing the config for a PostgreSQL connection.
@@ -67,4 +69,8 @@ export class PostgresConnection extends Connection {
             })
         }
     }
+
+    public schema(name?: string): Schema {
+        return new PostgresSchema(this, name)
+    }
 }

src/orm/dialect/PostgreSQLDialect.ts

@@ -1,9 +1,13 @@
 import {EscapeValue, QuerySafeValue, raw, SQLDialect} from './SQLDialect'
-import {Constraint, isConstraintGroup, isConstraintItem, SpecifiedField} from '../types'
+import {Constraint, inverseFieldType, isConstraintGroup, isConstraintItem, SpecifiedField} from '../types'
 import {AbstractBuilder} from '../builder/AbstractBuilder'
+import {ColumnBuilder, ConstraintBuilder, ConstraintType, IndexBuilder, TableBuilder} from '../schema/TableBuilder'
+import {ErrorWithContext, Maybe} from '../../util'
 
 /**
  * An implementation of the SQLDialect specific to PostgreSQL.
+ * @todo joins
+ * @todo sub-selects
  */
 export class PostgreSQLDialect extends SQLDialect {
 
@@ -29,7 +33,7 @@ export class PostgreSQLDialect extends SQLDialect {
                 `${pad(value.getSeconds())}`,
             ]
 
-            return new QuerySafeValue(value, `${y}-${m}-${d} ${h}:${i}:${s}`)
+            return new QuerySafeValue(value, `'${y}-${m}-${d} ${h}:${i}:${s}'`)
         } else if ( !isNaN(Number(value)) ) {
             return new QuerySafeValue(value, String(Number(value)))
         } else if ( value === null || typeof value === 'undefined' ) {
@@ -55,7 +59,7 @@ export class PostgreSQLDialect extends SQLDialect {
             'FROM (',
             ...query.split('\n').map(x => `  ${x}`),
             ') AS extollo_target_query',
-            `OFFSET ${start} LIMIT ${(end - start) + 1}`,
+            `OFFSET ${start} LIMIT ${(end - start) + 1}`,  // FIXME - the +1 is only needed when start === end
         ].join('\n')
     }
 
@@ -85,6 +89,11 @@ export class PostgreSQLDialect extends SQLDialect {
     }
 
     public renderSelect(builder: AbstractBuilder<any>): string {
+        const rawSql = builder.appliedRawSql
+        if ( rawSql ) {
+            return rawSql
+        }
+
         const indent = (item: string, level = 1) => Array(level + 1).fill('')
             .join('  ') + item
         const queryLines = [
@@ -147,6 +156,11 @@ export class PostgreSQLDialect extends SQLDialect {
 
     // TODO support FROM, RETURNING
     public renderUpdate(builder: AbstractBuilder<any>, data: {[key: string]: EscapeValue}): string {
+        const rawSql = builder.appliedRawSql
+        if ( rawSql ) {
+            return rawSql
+        }
+
         const queryLines: string[] = []
 
         // Add table source
@@ -171,6 +185,15 @@ export class PostgreSQLDialect extends SQLDialect {
     }
 
     public renderExistential(builder: AbstractBuilder<any>): string {
+        const rawSql = builder.appliedRawSql
+        if ( rawSql ) {
+            return `
+                SELECT EXISTS(
+                    ${rawSql}
+                )
+            `
+        }
+
         const query = builder.clone()
             .clearFields()
             .field(raw('TRUE'))
@@ -181,6 +204,11 @@ export class PostgreSQLDialect extends SQLDialect {
 
     // FIXME: subquery support here and with select
     public renderInsert(builder: AbstractBuilder<any>, data: {[key: string]: EscapeValue}|{[key: string]: EscapeValue}[] = []): string {
+        const rawSql = builder.appliedRawSql
+        if ( rawSql ) {
+            return rawSql
+        }
+
         const indent = (item: string, level = 1) => Array(level + 1).fill('')
             .join('  ') + item
         const queryLines: string[] = []
@@ -188,6 +216,11 @@ export class PostgreSQLDialect extends SQLDialect {
         if ( !Array.isArray(data) ) {
             data = [data]
         }
+
+        if ( data.length < 1 ) {
+            return ''
+        }
+
         const columns = Object.keys(data[0])
 
         // Add table source
@@ -227,6 +260,11 @@ export class PostgreSQLDialect extends SQLDialect {
     }
 
     public renderDelete(builder: AbstractBuilder<any>): string {
+        const rawSql = builder.appliedRawSql
+        if ( rawSql ) {
+            return rawSql
+        }
+
         const indent = (item: string, level = 1) => Array(level + 1).fill('')
             .join('  ') + item
         const queryLines: string[] = []
@@ -270,6 +308,11 @@ export class PostgreSQLDialect extends SQLDialect {
                 if ( isConstraintGroup(constraint) ) {
                     statements.push(`${indent}${statements.length < 1 ? '' : constraint.preop + ' '}(\n${constraintsToSql(constraint.items, level + 1)}\n${indent})`)
                 } else if ( isConstraintItem(constraint) ) {
+                    if ( Array.isArray(constraint.operand) && !constraint.operand.length ) {
+                        statements.push(`${indent}1 = 0  -- ${constraint.field} ${constraint.operator} empty set`)
+                        continue
+                    }
+
                     const field: string = constraint.field.split('.').map(x => `"${x}"`)
                         .join('.')
                     statements.push(`${indent}${statements.length < 1 ? '' : constraint.preop + ' '}${field} ${constraint.operator} ${this.escape(constraint.operand).value}`)
@@ -294,4 +337,247 @@ export class PostgreSQLDialect extends SQLDialect {
 
         return ['SET', ...sets].join('\n')
     }
+
+    public renderCreateTable(builder: TableBuilder): string {
+        const cols = this.renderTableColumns(builder).map(x => `    ${x}`)
+
+        const builderConstraints = builder.getConstraints()
+        const constraints: string[] = []
+        for ( const constraintName in builderConstraints ) {
+            if ( !Object.prototype.hasOwnProperty.call(builderConstraints, constraintName) ) {
+                continue
+            }
+
+            const constraintBuilder = builderConstraints[constraintName]
+            const constraintDefinition = this.renderConstraintDefinition(constraintBuilder)
+            if ( constraintDefinition ) {
+                constraints.push(`    CONSTRAINT ${constraintDefinition}`)
+            }
+        }
+
+        const parts = [
+            `CREATE TABLE ${builder.isSkippedIfExisting() ? 'IF NOT EXISTS ' : ''}${builder.name} (`,
+            [
+                ...cols,
+                ...constraints,
+            ].join(',\n'),
+            `)`,
+        ]
+
+        return parts.join('\n')
+    }
+
+    public renderTableColumns(builder: TableBuilder): string[] {
+        const defined = builder.getColumns()
+        const rendered: string[] = []
+
+        for ( const columnName in defined ) {
+            if ( !Object.prototype.hasOwnProperty.call(defined, columnName) ) {
+                continue
+            }
+
+            const columnBuilder = defined[columnName]
+            rendered.push(this.renderColumnDefinition(columnBuilder))
+        }
+
+        return rendered
+    }
+
+    /**
+     * Given a constraint schema-builder, render the constraint definition.
+     * @param builder
+     * @protected
+     */
+    protected renderConstraintDefinition(builder: ConstraintBuilder): Maybe<string> {
+        const constraintType = builder.getType()
+        if ( constraintType === ConstraintType.Unique ) {
+            const fields = builder.getFields()
+                .map(x => `"${x}"`)
+                .join(',')
+
+            return `${builder.name} UNIQUE(${fields})`
+        } else if ( constraintType === ConstraintType.Check ) {
+            const expression = builder.getExpression()
+            if ( !expression ) {
+                throw new ErrorWithContext('Cannot create check constraint without expression.', {
+                    constraintName: builder.name,
+                    tableName: builder.parent.name,
+                })
+            }
+
+            return `${builder.name} CHECK(${expression})`
+        }
+    }
+
+    /**
+     * Given a column-builder, render the SQL-definition as used in
+     * CREATE TABLE and ALTER TABLE statements.
+     * @fixme Type `serial` only exists on CREATE TABLE... queries
+     * @param builder
+     * @protected
+     */
+    protected renderColumnDefinition(builder: ColumnBuilder): string {
+        const type = builder.getType()
+        if ( !type ) {
+            throw new ErrorWithContext(`Missing field type for column: ${builder.name}`, {
+                columnName: builder.name,
+                columnType: type,
+            })
+        }
+
+        let render = `"${builder.name}"  ${inverseFieldType(type)}`
+
+        if ( builder.getLength() ) {
+            render += `(${builder.getLength()})`
+        }
+
+        const defaultValue = builder.getDefaultValue()
+        if ( typeof defaultValue !== 'undefined' ) {
+            render += ` DEFAULT ${this.escape(defaultValue)}`
+        }
+
+        if ( builder.isPrimary() ) {
+            render += ` CONSTRAINT ${builder.name}_pk PRIMARY KEY`
+        }
+
+        if ( builder.isUnique() ) {
+            render += ` UNIQUE`
+        }
+
+        render += ` ${builder.isNullable() ? 'NULL' : 'NOT NULL'}`
+        return render
+    }
+
+    public renderDropTable(builder: TableBuilder): string {
+        return `DROP TABLE ${builder.isSkippedIfExisting() ? 'IF EXISTS ' : ''}${builder.name}`
+    }
+
+    public renderCreateIndex(builder: IndexBuilder): string {
+        const cols = builder.getFields().map(x => `"${x}"`)
+        const parts = [
+            `CREATE ${builder.isUnique() ? 'UNIQUE ' : ''}INDEX ${builder.isSkippedIfExisting() ? 'IF NOT EXISTS ' : ''}${builder.name}`,
+            `    ON ${builder.parent.name}`,
+            `    (${cols.join(',')})`,
+        ]
+
+        return parts.join('\n')
+    }
+
+    public renderAlterTable(builder: TableBuilder): string {
+        const alters: string[] = []
+        const columns = builder.getColumns()
+
+        for ( const columnName in columns ) {
+            if ( !Object.prototype.hasOwnProperty.call(columns, columnName) ) {
+                continue
+            }
+
+            const columnBuilder = columns[columnName]
+            if ( !columnBuilder.isExisting() ) {
+                // The column doesn't exist on the table, but was added to the schema
+                alters.push(`    ADD COLUMN ${this.renderColumnDefinition(columnBuilder)}`)
+            } else if ( columnBuilder.isDirty() && columnBuilder.originalFromSchema ) {
+                // The column exists in the table, but was modified in the schema
+                if ( columnBuilder.isDropping() || columnBuilder.isDroppingIfExists() ) {
+                    alters.push(`    DROP COLUMN "${columnBuilder.name}"`)
+                    continue
+                }
+
+                // Change the data type of the column
+                if ( columnBuilder.getType() !== columnBuilder.originalFromSchema.getType() ) {
+                    const renderedType = `${columnBuilder.getType()}${columnBuilder.getLength() ? `(${columnBuilder.getLength()})` : ''}`
+                    alters.push(`    ALTER COLUMN "${columnBuilder.name}" TYPE ${renderedType}`)
+                }
+
+                // Change the default value of the column
+                if ( columnBuilder.getDefaultValue() !== columnBuilder.originalFromSchema.getDefaultValue() ) {
+                    alters.push(`    ALTER COLUMN "${columnBuilder.name}" SET default ${this.escape(columnBuilder.getDefaultValue())}`)
+                }
+
+                // Change the nullable-status of the column
+                if ( columnBuilder.isNullable() !== columnBuilder.originalFromSchema.isNullable() ) {
+                    if ( columnBuilder.isNullable() ) {
+                        alters.push(`    ALTER COLUMN "${columnBuilder.name}" DROP NOT NULL`)
+                    } else {
+                        alters.push(`    ALTER COLUMN "${columnBuilder.name}" SET NOT NULL`)
+                    }
+                }
+
+                // Change the name of the column
+                if ( columnBuilder.getRename() ) {
+                    alters.push(`    RENAME COLUMN "${columnBuilder.name}" TO "${columnBuilder.getRename()}"`)
+                }
+            }
+        }
+
+        const constraints = builder.getConstraints()
+        for ( const constraintName in constraints ) {
+            if ( !Object.prototype.hasOwnProperty.call(constraints, constraintName) ) {
+                continue
+            }
+
+            const constraintBuilder = constraints[constraintName]
+
+            // Drop the constraint if specified
+            if ( constraintBuilder.isDropping() ) {
+                alters.push(`    DROP CONSTRAINT ${constraintBuilder.name}`)
+                continue
+            }
+
+            // Drop the constraint with IF EXISTS if specified
+            if ( constraintBuilder.isDroppingIfExists() ) {
+                alters.push(`    DROP CONSTRAINT IF EXISTS ${constraintBuilder.name}`)
+                continue
+            }
+
+            // Otherwise, drop and recreate the constraint if it was modified
+            if ( constraintBuilder.isDirty() ) {
+                if ( constraintBuilder.isExisting() ) {
+                    alters.push(`    DROP CONSTRAINT IF EXISTS ${constraintBuilder.name}`)
+                }
+
+                const constraintDefinition = this.renderConstraintDefinition(constraintBuilder)
+                if ( constraintDefinition ) {
+                    alters.push(`    ADD CONSTRAINT ${constraintDefinition}`)
+                }
+            }
+        }
+
+        if ( builder.getRename() ) {
+            alters.push(`    RENAME TO "${builder.getRename()}"`)
+        }
+
+        return 'ALTER TABLE ' + builder.name + '\n' + alters.join(',\n')
+    }
+
+    public renderDropIndex(builder: IndexBuilder): string {
+        return `DROP INDEX ${builder.isDroppingIfExists() ? 'IF EXISTS ' : ''}${builder.name}`
+    }
+
+    public renderTransaction(queries: string[]): string {
+        const parts = [
+            'BEGIN',
+            ...queries,
+            'COMMIT',
+        ]
+
+        return parts.join(';\n\n')
+    }
+
+    public renderRenameIndex(builder: IndexBuilder): string {
+        return `ALTER INDEX ${builder.name} RENAME TO ${builder.getRename()}`
+    }
+
+    public renderRecreateIndex(builder: IndexBuilder): string {
+        return `${this.renderDropIndex(builder)};\n\n${this.renderCreateIndex(builder)}`
+    }
+
+    public renderDropColumn(builder: ColumnBuilder): string {
+        const parts = [
+            `ALTER TABLE ${builder.parent.name} ${builder.parent.isSkippedIfExisting() ? 'IF EXISTS ' : ''}`,
+            `    DROP COLUMN ${builder.isSkippedIfExisting() ? 'IF EXISTS ' : ''}${builder.name}`,
+        ]
+
+        return parts.join('\n')
+    }
 }

src/orm/dialect/SQLDialect.ts

@@ -1,6 +1,7 @@
 import {Constraint} from '../types'
 import {AbstractBuilder} from '../builder/AbstractBuilder'
 import {AppClass} from '../../lifecycle/AppClass'
+import {ColumnBuilder, IndexBuilder, TableBuilder} from '../schema/TableBuilder'
 
 /**
  * A value which can be escaped to be interpolated into an SQL query.
@@ -160,10 +161,141 @@ export abstract class SQLDialect extends AppClass {
      * This function should escape the values before they are included in the query string.
      *
      * @example
+     * ```ts
      * dialect.renderUpdateSet({field1: 'value', field2: 45})
      * // => "SET field1 = 'value', field2 = 45"
+     * ```
      *
      * @param data
      */
     public abstract renderUpdateSet(data: {[key: string]: EscapeValue}): string;
+
+    /**
+     * Given a table schema-builder, render a `CREATE TABLE...` query.
+     * @param builder
+     */
+    public abstract renderCreateTable(builder: TableBuilder): string;
+
+    /**
+     * Given a table schema-builder, render an `ALTER TABLE...` query.
+     * @param builder
+     */
+    public abstract renderAlterTable(builder: TableBuilder): string;
+
+    /**
+     * Given a table schema-builder, render a `DROP TABLE...` query.
+     * @param builder
+     */
+    public abstract renderDropTable(builder: TableBuilder): string;
+
+    /**
+     * Render the table-column definitions for the table defined by
+     * the given schema-builder.
+     *
+     * @example
+     * ```ts
+     * dialect.renderTableColumns(builder)
+     * // => ['col1 varchar(100) NULL', 'col2 serial NOT NULL']
+     * ```
+     *
+     * @param builder
+     */
+    public abstract renderTableColumns(builder: TableBuilder): string[];
+
+    /**
+     * Given an index schema-builder, render a `CREATE INDEX...` query.
+     * @param builder
+     */
+    public abstract renderCreateIndex(builder: IndexBuilder): string;
+
+    /**
+     * Given a column schema-builder, render an `ALTER TABLE... DROP COLUMN...` query.
+     * @param builder
+     */
+    public abstract renderDropColumn(builder: ColumnBuilder): string;
+
+    /**
+     * Given an index schema-builder, render a `DROP INDEX...` query.
+     * @param builder
+     */
+    public abstract renderDropIndex(builder: IndexBuilder): string;
+
+    /**
+     * Given an index schema-builder, render an `ALTER INDEX... RENAME...` query.
+     * @param builder
+     */
+    public abstract renderRenameIndex(builder: IndexBuilder): string;
+
+    /**
+     * Given an index schema-builder, render either an `ALTER INDEX...` query,
+     * or a `DROP INDEX...; CREATE INDEX...` query.
+     * @param builder
+     */
+    public abstract renderRecreateIndex(builder: IndexBuilder): string;
+
+    /**
+     * Given a series of fully-formed queries, render them as a single transaction.
+     * @example
+     * ```ts
+     * const queries = [
+     *  'SELECT * FROM a',
+     *  'UPDATE b SET col = 123',
+     * ]
+     *
+     * dialect.renderTransaction(queries)
+     * // => 'BEGIN; SELECT * FROM a; UPDATE b SET col = 123; COMMIT;'
+     * ```
+     * @param queries
+     */
+    public abstract renderTransaction(queries: string[]): string;
+
+    /**
+     * Given a table schema-builder, render a series of queries as a transaction
+     * that apply the given schema to database.
+     * @todo handle constraints better - ConstraintBuilder
+     * @param builder
+     */
+    public renderCommitSchemaTransaction(builder: TableBuilder): string {
+        if ( builder.isDropping() || builder.isDroppingIfExists() ) {
+            // If we're dropping the table, just return the DROP TABLE query
+            return this.renderTransaction([
+                this.renderDropTable(builder),
+            ])
+        }
+
+        // Render the queries to create/update/drop indexes
+        const indexes = Object.values(builder.getIndexes())
+            .filter(index => !index.isExisting() || index.isDirty())
+            .map(index => {
+                if ( index.isDropping() || index.isDroppingIfExists() ) {
+                    return this.renderDropIndex(index)
+                }
+
+                if ( index.isExisting() ) {
+                    // The index was changed in the schema, but exists in the DB
+                    return this.renderRecreateIndex(index)
+                }
+
+                return this.renderCreateIndex(index)
+            })
+
+        // Render the queries to rename indexes AFTER the above operations
+        const renamedIndexes = Object.values(builder.getIndexes())
+            .filter(idx => idx.getRename())
+            .map(x => this.renderRenameIndex(x))
+
+        let parts: string[] = []
+
+        // Render the CREATE/ALTER TABLE query
+        if ( !builder.isExisting() && builder.isDirty() ) {
+            parts.push(this.renderCreateTable(builder))
+        } else if ( builder.isExisting() && builder.isDirty() ) {
+            parts.push(this.renderAlterTable(builder))
+        }
+
+        // Render the various schema queries as a single transaction
+        parts = parts.concat(...indexes)
+        parts = parts.concat(...renamedIndexes)
+        return this.renderTransaction(parts)
+    }
 }

src/orm/directive/CreateMigrationDirective.ts

@@ -0,0 +1,48 @@
+import {Directive, OptionDefinition} from '../../cli'
+import {Injectable} from '../../di'
+import {stringToPascal} from '../../util'
+import {templateMigration} from '../template/migration'
+
+/**
+ * CLI directive that creates migration classes from template.
+ */
+@Injectable()
+export class CreateMigrationDirective extends Directive {
+    getDescription(): string {
+        return 'create a new migration'
+    }
+
+    getKeywords(): string | string[] {
+        return ['create-migration', 'make-migration']
+    }
+
+    getOptions(): OptionDefinition[] {
+        return [
+            '{description} | Description of what the migration does',
+        ]
+    }
+
+    getHelpText(): string {
+        return [
+            'Creates a new migration file in `src/app/migrations`.',
+            'To use, specify a string describing what the migration does. For example:',
+            './ex create-migration "Add version column to sessions table"',
+        ].join('\n\n')
+    }
+
+    async handle(): Promise<void> {
+        const description = this.option('description')
+        const className = `${stringToPascal(description)}Migration`
+        const fileName = `${(new Date()).toISOString()}_${className}.migration.ts`
+        const path = this.app().path('..', 'src', 'app', 'migrations', fileName)
+
+        // Create the migrations directory, if it doesn't already exist
+        await path.concat('..').mkdir()
+
+        // Render the template
+        const rendered = await templateMigration.render(className, className, path)
+        await path.write(rendered)
+
+        this.success(`Created migration: ${className}`)
+    }
+}

src/orm/directive/MigrateDirective.ts

@@ -0,0 +1,117 @@
+import {Directive, OptionDefinition} from '../../cli'
+import {Container, Inject, Injectable} from '../../di'
+import {EventBus} from '../../event/EventBus'
+import {Migrator} from '../migrations/Migrator'
+import {Migrations} from '../services/Migrations'
+import {ApplyingMigrationEvent} from '../migrations/events/ApplyingMigrationEvent'
+import {AppliedMigrationEvent} from '../migrations/events/AppliedMigrationEvent'
+import {EventSubscription} from '../../event/types'
+import {NothingToMigrateError} from '../migrations/NothingToMigrateError'
+
+/**
+ * CLI directive that applies migrations using the default Migrator.
+ * @fixme Support dry run mode
+ */
+@Injectable()
+export class MigrateDirective extends Directive {
+    @Inject()
+    protected readonly bus!: EventBus
+
+    @Inject('injector')
+    protected readonly injector!: Container
+
+    /** Event bus subscriptions. */
+    protected subscriptions: EventSubscription[] = []
+
+    getKeywords(): string | string[] {
+        return ['migrate']
+    }
+
+    getDescription(): string {
+        return 'apply pending migrations'
+    }
+
+    getOptions(): OptionDefinition[] {
+        return [
+            '--package -p {name} | apply migrations for a specific namespace',
+            '--identifier -i {name} | apply a specific migration, by identifier',
+        ]
+    }
+
+    getHelpText(): string {
+        return [
+            'Migrations are single-run code patches used to track changes to things like database schemata.',
+            '',
+            'You can create migrations in your app using the ./ex command and they can be applied and rolled-back.',
+            '',
+            './ex migrate:create "Add version column to sessions table"',
+            '',
+            'Modules and packages can also register their own migrations. These are run by default.',
+            '',
+            'To run the migrations for a specific package, and no others, use the --package option. Example:',
+            '',
+            './ex migrate --package @extollo',
+            '',
+        ].join('\n')
+    }
+
+    async handle(): Promise<void> {
+        await this.registerListeners()
+
+        const namespace = this.option('package')
+        const identifier = this.option('identifier')
+
+        let identifiers
+        if ( namespace ) {
+            identifiers = (this.injector.make<Migrations>(Migrations))
+                .all(namespace)
+                .map(id => `${namespace}:${id}`)
+        }
+
+        if ( identifier ) {
+            if ( !identifiers ) {
+                identifiers = [identifier]
+            }
+
+            identifiers = identifiers.filter(x => x === identifier)
+        }
+
+        let error
+        try {
+            await (this.injector.make<Migrator>(Migrator)).migrate(identifiers)
+        } catch (e) {
+            if ( e instanceof NothingToMigrateError ) {
+                this.info(e.message)
+            } else {
+                error = e
+                this.error(e)
+            }
+        } finally {
+            await this.removeListeners()
+        }
+
+        if ( error ) {
+            throw error
+        }
+    }
+
+    /**
+     * Register event bus listeners to print messages for the user.
+     * @protected
+     */
+    protected async registerListeners(): Promise<void> {
+        this.subscriptions.push(await this.bus.subscribe(ApplyingMigrationEvent, event => {
+            this.info(`Applying migration ${event.migration.identifier}...`)
+        }))
+
+        this.subscriptions.push(await this.bus.subscribe(AppliedMigrationEvent, event => {
+            this.success(`Applied migration: ${event.migration.identifier}`)
+        }))
+    }
+
+    /** Remove event bus listeners before finish. */
+    protected async removeListeners(): Promise<void> {
+        await Promise.all(this.subscriptions.map(x => x.unsubscribe()))
+        this.subscriptions = []
+    }
+}

src/orm/directive/RollbackDirective.ts

@@ -0,0 +1,102 @@
+import {Directive, OptionDefinition} from '../../cli'
+import {Container, Inject, Injectable} from '../../di'
+import {EventBus} from '../../event/EventBus'
+import {Migrator} from '../migrations/Migrator'
+import {Migrations} from '../services/Migrations'
+import {RollingBackMigrationEvent} from '../migrations/events/RollingBackMigrationEvent'
+import {RolledBackMigrationEvent} from '../migrations/events/RolledBackMigrationEvent'
+import {EventSubscription} from '../../event/types'
+import {NothingToMigrateError} from '../migrations/NothingToMigrateError'
+
+/**
+ * CLI directive that undoes applied migrations using the default Migrator.
+ * @fixme Support dry run mode
+ */
+@Injectable()
+export class RollbackDirective extends Directive {
+    @Inject()
+    protected readonly bus!: EventBus
+
+    @Inject('injector')
+    protected readonly injector!: Container
+
+    @Inject()
+    protected readonly migrations!: Migrations
+
+    /** Event bus subscriptions. */
+    protected subscriptions: EventSubscription[] = []
+
+    getKeywords(): string | string[] {
+        return ['rollback']
+    }
+
+    getDescription(): string {
+        return 'roll-back applied migrations'
+    }
+
+    getOptions(): OptionDefinition[] {
+        return [
+            '--identifier -i {name} | roll-back a specific migration, by identifier',
+        ]
+    }
+
+    getHelpText(): string {
+        return [
+            'Use this command to undo one or more migrations that were applied.',
+            '',
+            'By default, the command will undo all of the migrations applied the last time the migrate command was run.',
+            '',
+            'To undo a specific migration, pass its identifier using the --identifier option.',
+            '',
+        ].join('\n')
+    }
+
+    async handle(): Promise<void> {
+        await this.registerListeners()
+
+        const identifier = this.option('identifier')
+
+        let identifiers
+        if ( identifier ) {
+            identifiers = [identifier]
+        }
+
+        let error
+        try {
+            await (this.injector.make<Migrator>(Migrator)).rollback(identifiers)
+        } catch (e) {
+            if ( e instanceof NothingToMigrateError ) {
+                this.info(e.message)
+            } else {
+                error = e
+                this.error(e)
+            }
+        } finally {
+            await this.removeListeners()
+        }
+
+        if ( error ) {
+            throw error
+        }
+    }
+
+    /**
+     * Register event-bus listeners to print messages for the user.
+     * @protected
+     */
+    protected async registerListeners(): Promise<void> {
+        this.subscriptions.push(await this.bus.subscribe(RollingBackMigrationEvent, event => {
+            this.info(`Rolling-back migration ${event.migration.identifier}...`)
+        }))
+
+        this.subscriptions.push(await this.bus.subscribe(RolledBackMigrationEvent, event => {
+            this.success(`Rolled-back migration: ${event.migration.identifier}`)
+        }))
+    }
+
+    /** Remove event bus listeners before finish. */
+    protected async removeListeners(): Promise<void> {
+        await Promise.all(this.subscriptions.map(x => x.unsubscribe()))
+        this.subscriptions = []
+    }
+}

src/orm/index.ts

@@ -18,9 +18,6 @@ export * from './model/ModelResultIterable'
 export * from './model/events'
 export * from './model/Model'
 
-export * from './services/Database'
-export * from './services/Models'
-
 export * from './support/SessionModel'
 export * from './support/ORMSession'
 export * from './support/CacheModel'
@@ -28,3 +25,25 @@ export * from './support/ORMCache'
 
 export * from './DatabaseService'
 export * from './types'
+
+export * from './schema/TableBuilder'
+export * from './schema/Schema'
+export * from './schema/PostgresSchema'
+
+export * from './migrations/NothingToMigrateError'
+export * from './migrations/events/ApplyingMigrationEvent'
+export * from './migrations/events/AppliedMigrationEvent'
+export * from './migrations/events/RollingBackMigrationEvent'
+export * from './migrations/events/RolledBackMigrationEvent'
+export * from './migrations/Migration'
+export * from './migrations/Migrator'
+export * from './migrations/MigratorFactory'
+export * from './migrations/DatabaseMigrator'
+
+export * from './services/Database'
+export * from './services/Models'
+export * from './services/Migrations'
+
+export * from './directive/CreateMigrationDirective'
+export * from './directive/MigrateDirective'
+export * from './directive/RollbackDirective'

src/orm/migrations/DatabaseMigrator.ts

@@ -0,0 +1,179 @@
+import {Container, Inject, Injectable} from '../../di'
+import {Migrator} from './Migrator'
+import {DatabaseService} from '../DatabaseService'
+import {FieldType} from '../types'
+import {Migration} from './Migration'
+import {Builder} from '../builder/Builder'
+
+/**
+ * Migrator implementation that tracks applied migrations in a database table.
+ * @todo allow configuring more of this
+ */
+@Injectable()
+export class DatabaseMigrator extends Migrator {
+    @Inject()
+    protected readonly db!: DatabaseService
+
+    @Inject('injector')
+    protected readonly injector!: Container
+
+    /** True if we've initialized the migrator. */
+    protected initialized = false
+
+    public async initialize(): Promise<void> {
+        await super.initialize()
+
+        if ( this.initialized ) {
+            return
+        }
+
+        const schema = this.db.get().schema()
+        if ( !(await schema.hasTable('migrations')) ) {
+            const table = await schema.table('migrations')
+
+            table.primaryKey('id', FieldType.serial).required()
+
+            table.column('identifier')
+                .type(FieldType.varchar)
+                .required()
+
+            table.column('applygroup')
+                .type(FieldType.integer)
+                .required()
+
+            table.column('applydate')
+                .type(FieldType.timestamp)
+                .required()
+
+            await schema.commit(table)
+        }
+
+        this.initialized = true
+    }
+
+    async has(migration: Migration): Promise<boolean> {
+        return this.builder()
+            .connection('default')
+            .select('id')
+            .from('migrations')
+            .where('identifier', '=', migration.identifier)
+            .exists()
+    }
+
+    async markApplied(migrations: Migration | Migration[], applyDate: Date = new Date()): Promise<void> {
+        if ( !Array.isArray(migrations) ) {
+            migrations = [migrations]
+        }
+
+        const applyGroup = await this.getNextGroupIdentifier()
+        const rows = migrations.map(migration => {
+            return {
+                applygroup: applyGroup,
+                applydate: applyDate,
+                identifier: migration.identifier,
+            }
+        })
+
+        await this.builder()
+            .connection('default')
+            .table('migrations')
+            .insert(rows)
+    }
+
+    async unmarkApplied(migrations: Migration | Migration[]): Promise<void> {
+        if ( !Array.isArray(migrations) ) {
+            migrations = [migrations]
+        }
+
+        const identifiers = migrations.map(migration => migration.identifier)
+
+        await this.builder()
+            .connection('default')
+            .table('migrations')
+            .whereIn('identifier', identifiers)
+            .delete()
+    }
+
+    async getLastApplyGroup(): Promise<string[]> {
+        const applyGroup = await this.builder()
+            .connection('default')
+            .select('applygroup')
+            .from('migrations')
+            .get()
+            .max<number>('applygroup')
+
+        return this.builder()
+            .connection('default')
+            .select('identifier')
+            .from('migrations')
+            .where('applygroup', '=', applyGroup)
+            .get()
+            .asyncPipe()
+            .tap(coll => {
+                return coll.pluck<string>('identifier')
+            })
+            .tap(coll => {
+                return coll.all()
+            })
+            .resolve()
+    }
+
+    /**
+     * Helper method to look up the next `applygroup` that should be used.
+     * @protected
+     */
+    protected async getNextGroupIdentifier(): Promise<number> {
+        const current = await this.builder()
+            .connection('default')
+            .select('applygroup')
+            .from('migrations')
+            .get()
+            .max<number>('applygroup')
+
+        return (current ?? 0) + 1
+    }
+
+    /**
+     * Given a list of migration identifiers, filter out those that have been applied.
+     * @override to make this more efficient
+     * @param identifiers
+     * @protected
+     */
+    protected async filterAppliedMigrations(identifiers: string[]): Promise<string[]> {
+        const existing = await this.builder()
+            .connection('default')
+            .select('identifier')
+            .from('migrations')
+            .whereIn('identifier', identifiers)
+            .get()
+            .pluck<string>('identifier')
+
+        return identifiers.filter(id => !existing.includes(id))
+    }
+
+    /**
+     * Given a list of migration identifiers, filter out those that have not been applied.
+     * @override to make this more efficient
+     * @param identifiers
+     * @protected
+     */
+    protected async filterPendingMigrations(identifiers: string[]): Promise<string[]> {
+        const existing = await this.builder()
+            .connection('default')
+            .select('identifier')
+            .from('migrations')
+            .whereIn('identifier', identifiers)
+            .get()
+            .pluck<string>('identifier')
+
+        return existing.all()
+    }
+
+    /**
+     * Get a query builder instance.
+     * @protected
+     */
+    protected builder(): Builder {
+        return this.injector.make<Builder>(Builder)
+    }
+}

src/orm/migrations/Migration.ts

@@ -0,0 +1,39 @@
+import {Injectable} from '../../di'
+import {Awaitable} from '../../util'
+
+/**
+ * Abstract base-class for one-time migrations.
+ */
+@Injectable()
+export abstract class Migration {
+    /** Set by the Migrations unit on load. */
+    protected migrationIdentifier!: string
+
+    /**
+     * Sets the migration identifier.
+     * This is used internally when the Migrations service loads
+     * the migration files to determine the ID from the file-name.
+     * It shouldn't be used externally.
+     * @param name
+     */
+    public setMigrationIdentifier(name: string): void {
+        this.migrationIdentifier = name
+    }
+
+    /**
+     * Get the unique identifier of this migration.
+     */
+    public get identifier(): string {
+        return this.migrationIdentifier
+    }
+
+    /**
+     * Apply the migration.
+     */
+    abstract up(): Awaitable<void>
+
+    /**
+     * Undo the migration.
+     */
+    abstract down(): Awaitable<void>
+}

src/orm/migrations/Migrator.ts

@@ -0,0 +1,295 @@
+import {Container, Inject, Injectable} from '../../di'
+import {Awaitable, collect, ErrorWithContext} from '../../util'
+import {Migration} from './Migration'
+import {Migrations} from '../services/Migrations'
+import {EventBus} from '../../event/EventBus'
+import {ApplyingMigrationEvent} from './events/ApplyingMigrationEvent'
+import {AppliedMigrationEvent} from './events/AppliedMigrationEvent'
+import {RollingBackMigrationEvent} from './events/RollingBackMigrationEvent'
+import {RolledBackMigrationEvent} from './events/RolledBackMigrationEvent'
+import {NothingToMigrateError} from './NothingToMigrateError'
+
+/**
+ * Manages single-run patches/migrations.
+ */
+@Injectable()
+export abstract class Migrator {
+    @Inject()
+    protected readonly migrations!: Migrations
+
+    @Inject()
+    protected readonly bus!: EventBus
+
+    @Inject('injector')
+    protected readonly injector!: Container
+
+    /**
+     * Should resolve true if the given migration has already been applied.
+     * @param migration
+     */
+    public abstract has(migration: Migration): Awaitable<boolean>
+
+    /**
+     * Should mark the given migrations as being applied.
+     *
+     * If a date is specified, then that is the timestamp when the migrations
+     * were applied, otherwise, use `new Date()`.
+     *
+     * @param migrations
+     * @param date
+     */
+    public abstract markApplied(migrations: Migration | Migration[], date?: Date): Awaitable<void>
+
+    /**
+     * Should un-mark the given migrations as being applied.
+     * @param migration
+     */
+    public abstract unmarkApplied(migration: Migration | Migration[]): Awaitable<void>
+
+    /**
+     * Get the identifiers of the last group of migrations that were applied.
+     */
+    public abstract getLastApplyGroup(): Awaitable<string[]>
+
+    /**
+     * Do any initial setup required to get the migrator ready.
+     * This can be overridden by implementation classes to do any necessary setup.
+     */
+    public initialize(): Awaitable<void> {}  // eslint-disable-line @typescript-eslint/no-empty-function
+
+    /**
+     * Apply pending migrations.
+     *
+     * If identifiers are specified, only the pending migrations with those
+     * identifiers are applied. If none are specified, all pending migrations
+     * will be applied.
+     *
+     * @param identifiers
+     */
+    public async migrate(identifiers?: string[]): Promise<void> {
+        await this.initialize()
+
+        if ( !identifiers ) {
+            identifiers = this.getAllMigrationIdentifiers()
+        }
+
+        identifiers = (await this.filterAppliedMigrations(identifiers)).sort()
+        if ( !identifiers.length ) {
+            throw new NothingToMigrateError()
+        }
+
+        const migrations = collect(identifiers)
+            .map(id => {
+                const migration = this.migrations.get(id)
+
+                if ( !migration ) {
+                    throw new ErrorWithContext(`Unable to find migration with identifier: ${id}`, {
+                        identifier: id,
+                    })
+                }
+
+                return migration
+            })
+
+        await migrations.promiseMap(migration => {
+            return this.apply(migration)
+        })
+
+        await this.markApplied(migrations.all())
+    }
+
+    /**
+     * Rollback applied migrations.
+     *
+     * If specified, only applied migrations with the given identifiers will
+     * be rolled back. If not specified, then the last "batch" of applied
+     * migrations will be rolled back.
+     *
+     * @param identifiers
+     */
+    public async rollback(identifiers?: string[]): Promise<void> {
+        await this.initialize()
+
+        if ( !identifiers ) {
+            identifiers = await this.getLastApplyGroup()
+        }
+
+        identifiers = (await this.filterPendingMigrations(identifiers)).sort()
+        if ( !identifiers.length ) {
+            throw new NothingToMigrateError()
+        }
+
+        const migrations = collect(identifiers)
+            .map(id => {
+                const migration = this.migrations.get(id)
+
+                if ( !migration ) {
+                    throw new ErrorWithContext(`Unable to find migration with identifier: ${id}`, {
+                        identifier: id,
+                    })
+                }
+
+                return migration
+            })
+
+        await migrations.promiseMap(migration => {
+            return this.undo(migration)
+        })
+
+        await this.unmarkApplied(migrations.all())
+    }
+
+    /**
+     * Apply a single migration.
+     * @param migration
+     */
+    public async apply(migration: Migration): Promise<void> {
+        await this.initialize()
+
+        await this.applying(migration)
+
+        await migration.up()
+
+        await this.applied(migration)
+    }
+
+    /**
+     * Rollback a single migration.
+     * @param migration
+     */
+    public async undo(migration: Migration): Promise<void> {
+        await this.initialize()
+
+        await this.rollingBack(migration)
+
+        await migration.down()
+
+        await this.rolledBack(migration)
+    }
+
+    /**
+     * Get all registered migrations, by their string-form identifiers.
+     * @protected
+     */
+    protected getAllMigrationIdentifiers(): string[] {
+        return collect<string>(this.migrations.namespaces())
+            .map(nsp => {
+                return this.migrations.all(nsp)
+                    .map(id => `${nsp}:${id}`)
+            })
+            .tap(coll => {
+                // non-namespaced migrations
+                coll.push(this.migrations.all())
+                return coll
+            })
+            .reduce((current, item) => {
+                return current.concat(item)
+            }, [])
+    }
+
+    /**
+     * Given a list of migration identifiers, filter out those that have been applied.
+     * @param identifiers
+     * @protected
+     */
+    protected async filterAppliedMigrations(identifiers: string[]): Promise<string[]> {
+        return collect(identifiers)
+            .partialMap(identifier => {
+                const migration = this.migrations.get(identifier)
+                if ( migration ) {
+                    return {
+                        identifier,
+                        migration,
+                    }
+                }
+            })
+            .asyncPipe()
+            .tap(coll => {
+                return coll.promiseMap(async group => {
+                    return {
+                        ...group,
+                        has: await this.has(group.migration),
+                    }
+                })
+            })
+            .tap(coll => {
+                return coll.filter(group => !group.has)
+                    .pluck<string>('identifier')
+                    .all()
+            })
+            .resolve()
+    }
+
+    /**
+     * Given a list of migration identifiers, filter out those that have not been applied.
+     * @param identifiers
+     * @protected
+     */
+    protected async filterPendingMigrations(identifiers: string[]): Promise<string[]> {
+        return collect(identifiers)
+            .partialMap(identifier => {
+                const migration = this.migrations.get(identifier)
+                if ( migration ) {
+                    return {
+                        identifier,
+                        migration,
+                    }
+                }
+            })
+            .asyncPipe()
+            .tap(coll => {
+                return coll.promiseMap(async group => {
+                    return {
+                        ...group,
+                        has: await this.has(group.migration),
+                    }
+                })
+            })
+            .tap(coll => {
+                return coll.filter(group => group.has)
+                    .pluck<string>('identifier')
+                    .all()
+            })
+            .resolve()
+    }
+
+    /**
+     * Fire the ApplyingMigrationEvent.
+     * @param migration
+     * @protected
+     */
+    protected async applying(migration: Migration): Promise<void> {
+        const event = <ApplyingMigrationEvent> this.injector.make(ApplyingMigrationEvent, migration)
+        await this.bus.dispatch(event)
+    }
+
+    /**
+     * Fire the AppliedMigrationEvent.
+     * @param migration
+     * @protected
+     */
+    protected async applied(migration: Migration): Promise<void> {
+        const event = <AppliedMigrationEvent> this.injector.make(AppliedMigrationEvent, migration)
+        await this.bus.dispatch(event)
+    }
+
+    /**
+     * Fire the RollingBackMigrationEvent.
+     * @param migration
+     * @protected
+     */
+    protected async rollingBack(migration: Migration): Promise<void> {
+        const event = <RollingBackMigrationEvent> this.injector.make(RollingBackMigrationEvent, migration)
+        await this.bus.dispatch(event)
+    }
+
+    /**
+     * Fire the RolledBackMigrationEvent.
+     * @param migration
+     * @protected
+     */
+    protected async rolledBack(migration: Migration): Promise<void> {
+        const event = <RolledBackMigrationEvent> this.injector.make(RolledBackMigrationEvent, migration)
+        await this.bus.dispatch(event)
+    }
+}

src/orm/migrations/MigratorFactory.ts

@@ -0,0 +1,81 @@
+import {
+    AbstractFactory,
+    DependencyRequirement,
+    PropertyDependency,
+    isInstantiable,
+    DEPENDENCY_KEYS_METADATA_KEY,
+    DEPENDENCY_KEYS_PROPERTY_METADATA_KEY, Instantiable, Injectable, Inject,
+} from '../../di'
+import {Collection, ErrorWithContext} from '../../util'
+import {Logging} from '../../service/Logging'
+import {Config} from '../../service/Config'
+import {Migrator} from './Migrator'
+import {DatabaseMigrator} from './DatabaseMigrator'
+
+/**
+ * A dependency injection factory that matches the abstract Migrator class
+ * and produces an instance of the configured session driver implementation.
+ */
+@Injectable()
+export class MigratorFactory extends AbstractFactory<Migrator> {
+    @Inject()
+    protected readonly logging!: Logging
+
+    @Inject()
+    protected readonly config!: Config
+
+    constructor() {
+        super({})
+    }
+
+    produce(): Migrator {
+        return new (this.getMigratorClass())()
+    }
+
+    match(something: unknown): boolean {
+        return something === Migrator
+    }
+
+    getDependencyKeys(): Collection<DependencyRequirement> {
+        const meta = Reflect.getMetadata(DEPENDENCY_KEYS_METADATA_KEY, this.getMigratorClass())
+        if ( meta ) {
+            return meta
+        }
+
+        return new Collection<DependencyRequirement>()
+    }
+
+    getInjectedProperties(): Collection<PropertyDependency> {
+        const meta = new Collection<PropertyDependency>()
+        let currentToken = this.getMigratorClass()
+
+        do {
+            const loadedMeta = Reflect.getMetadata(DEPENDENCY_KEYS_PROPERTY_METADATA_KEY, currentToken)
+            if ( loadedMeta ) {
+                meta.concat(loadedMeta)
+            }
+            currentToken = Object.getPrototypeOf(currentToken)
+        } while (Object.getPrototypeOf(currentToken) !== Function.prototype && Object.getPrototypeOf(currentToken) !== Object.prototype)
+
+        return meta
+    }
+
+    /**
+     * Return the instantiable class of the configured migrator backend.
+     * @protected
+     * @return Instantiable<Migrator>
+     */
+    protected getMigratorClass(): Instantiable<Migrator> {
+        const MigratorClass = this.config.get('database.migrations.driver', DatabaseMigrator)
+
+        if ( !isInstantiable(MigratorClass) || !(MigratorClass.prototype instanceof Migrator) ) {
+            const e = new ErrorWithContext('Provided migration driver class does not extend from @extollo/lib.Migrator')
+            e.context = {
+                configKey: 'database.migrations.driver',
+                class: MigratorClass.toString(),
+            }
+        }
+
+        return MigratorClass
+    }
+}

src/orm/migrations/NothingToMigrateError.ts

@@ -0,0 +1,14 @@
+import {ErrorWithContext} from '../../util'
+
+/**
+ * Error thrown when the migrator is run, but no migrations need
+ * to be applied/rolled-back.
+ */
+export class NothingToMigrateError extends ErrorWithContext {
+    constructor(
+        message = 'There is nothing to migrate',
+        context?: {[key: string]: any},
+    ) {
+        super(message, context)
+    }
+}

src/orm/migrations/events/AppliedMigrationEvent.ts

@@ -0,0 +1,8 @@
+import {Injectable} from '../../../di'
+import {MigrationEvent} from './MigrationEvent'
+
+/**
+ * Event fired after a migration is applied.
+ */
+@Injectable()
+export class AppliedMigrationEvent extends MigrationEvent {}

src/orm/migrations/events/ApplyingMigrationEvent.ts

@@ -0,0 +1,8 @@
+import {Injectable} from '../../../di'
+import {MigrationEvent} from './MigrationEvent'
+
+/**
+ * Event fired before a migration is applied.
+ */
+@Injectable()
+export class ApplyingMigrationEvent extends MigrationEvent {}

src/orm/migrations/events/MigrationEvent.ts

@@ -0,0 +1,49 @@
+import {Event} from '../../../event/Event'
+import {Migration} from '../Migration'
+import {Inject, Injectable} from '../../../di'
+import {Migrations} from '../../services/Migrations'
+import {ErrorWithContext} from '../../../util'
+
+/**
+ * Generic base-class for migration-related events.
+ */
+@Injectable()
+export abstract class MigrationEvent extends Event {
+    @Inject()
+    protected readonly migrations!: Migrations
+
+    /** The migration relevant to this event. */
+    private internalMigration: Migration
+
+    /**
+     * Get the relevant migration.
+     */
+    public get migration(): Migration {
+        return this.internalMigration
+    }
+
+    constructor(
+        migration: Migration,
+    ) {
+        super()
+        this.internalMigration = migration
+    }
+
+    dehydrate(): {identifier: string} {
+        return {
+            identifier: this.migration.identifier,
+        }
+    }
+
+    rehydrate(state: {identifier: string}): void {
+        const migration = this.migrations.get(state.identifier)
+
+        if ( !migration ) {
+            throw new ErrorWithContext(`Unable to find migration with identifier: ${state.identifier}`, {
+                identifier: state.identifier,
+            })
+        }
+
+        this.internalMigration = migration
+    }
+}

src/orm/migrations/events/RolledBackMigrationEvent.ts

@@ -0,0 +1,8 @@
+import {Injectable} from '../../../di'
+import {MigrationEvent} from './MigrationEvent'
+
+/**
+ * Event fired after a migration has been rolled-back.
+ */
+@Injectable()
+export class RolledBackMigrationEvent extends MigrationEvent {}

src/orm/migrations/events/RollingBackMigrationEvent.ts

@@ -0,0 +1,8 @@
+import {Injectable} from '../../../di'
+import {MigrationEvent} from './MigrationEvent'
+
+/**
+ * Event fired before a migration is rolled back.
+ */
+@Injectable()
+export class RollingBackMigrationEvent extends MigrationEvent {}

src/orm/schema/PostgresSchema.ts

@@ -0,0 +1,345 @@
+import {Schema} from './Schema'
+import {Awaitable, collect, Collection} from '../../util'
+import {ConstraintType, TableBuilder} from './TableBuilder'
+import {PostgresConnection} from '../connection/PostgresConnection'
+import {Builder} from '../builder/Builder'
+import {raw} from '../dialect/SQLDialect'
+import {QueryRow} from '../types'
+
+/**
+ * A PostgreSQL-compatible schema implementation.
+ */
+export class PostgresSchema extends Schema {
+    constructor(
+        connection: PostgresConnection,
+        public readonly schema: string = 'public',
+    ) {
+        super(connection)
+    }
+
+    hasColumn(table: string, name: string): Awaitable<boolean> {
+        return (new Builder()).connection(this.connection)
+            .select(raw('*'))
+            .from('information_schema.columns')
+            .where('table_schema', '=', this.schema)
+            .where('table_name', '=', table)
+            .where('column_name', '=', name)
+            .exists()
+    }
+
+    async hasColumns(table: string, name: string[]): Promise<boolean> {
+        const num = await (new Builder()).connection(this.connection)
+            .select(raw('*'))
+            .from('information_schema.columns')
+            .where('table_schema', '=', this.schema)
+            .where('table_name', '=', table)
+            .whereIn('column_name', name)
+            .get()
+            .count()
+
+        return num === name.length
+    }
+
+    hasTable(name: string): Awaitable<boolean> {
+        return (new Builder()).connection(this.connection)
+            .select(raw('*'))
+            .from('information_schema.tables')
+            .where('table_schema', '=', this.schema)
+            .where('table_name', '=', name)
+            .exists()
+    }
+
+    async table(table: string): Promise<TableBuilder> {
+        return this.populateTable(new TableBuilder(table))
+    }
+
+    /**
+     * If the table for the given TableBuilder already exists in the
+     * database, fill in the columns, constraints, and indexes.
+     * @param table
+     * @protected
+     */
+    protected async populateTable(table: TableBuilder): Promise<TableBuilder> {
+        if ( await this.hasTable(table.name) ) {
+            // Load the existing columns
+            const cols = await this.getColumns(table.name)
+            cols.each(col => {
+                table.column(col.column_name)
+                    .type(col.data_type)
+                    .pipe()
+                    .when(col.is_nullable, builder => {
+                        builder.isNullable()
+                        return builder
+                    })
+                    .when(col.column_default, builder => {
+                        builder.default(raw(col.column_default))
+                        return builder
+                    })
+            })
+
+            // Load the existing constraints
+            const constraints = await this.getConstraints(table.name)
+
+            // Apply the unique constraints
+            const uniques = constraints.where('constraint_type', '=', 'u')
+                .sortBy('constraint_name')
+                .groupBy('constraint_name')
+
+            for ( const key in uniques ) {
+                if ( !Object.prototype.hasOwnProperty.call(uniques, key) ) {
+                    continue
+                }
+
+                table.constraint(key)
+                    .type(ConstraintType.Unique)
+                    .pipe()
+                    .peek(constraint => {
+                        collect<{column_name: string}>(uniques[key])  // eslint-disable-line camelcase
+                            .pluck<string>('column_name')
+                            .each(column => constraint.field(column))
+                    })
+                    .get()
+                    .flagAsExistingInSchema()
+            }
+
+            // Apply the primary key constraints
+            constraints.where('constraint_type', '=', 'p')
+                .pipe()
+                .when(c => c.count() > 0, pk => {
+                    pk.each(constraint => {
+                        table.column(constraint.column_name)
+                            .primary()
+                    })
+
+                    return pk
+                })
+
+            // Apply the non-null constraints
+            // Builder columns are non-null by default, so mark the others as nullable
+            const nonNullable = constraints.filter(x => !x.constraint_type)
+                .where('is_nullable', '=', 'NO')
+
+            collect<string>(Object.keys(table.getColumns()))
+                .map(column => {
+                    return {
+                        column,
+                    }
+                })
+                .whereNotIn('column', nonNullable.pluck('column_name'))
+                .pluck<string>('column')
+                .each(column => {
+                    table.column(column)
+                        .nullable()
+                })
+
+            // Look up and apply the check constraints
+            const checkConstraints = await this.getCheckConstraints(table.name)
+
+            checkConstraints.each(constraint => {
+                table.constraint(constraint.constraint_name)
+                    .type(ConstraintType.Check)
+                    .expression(constraint.check_clause)
+                    .flagAsExistingInSchema()
+            })
+
+            // Mark the columns as existing in the database
+            cols.each(col => {
+                table.column(col.column_name)
+                    .flagAsExistingInSchema()
+            })
+
+            // Look up table indexes
+            const indexes = await this.getIndexes(table.name)
+            const groupedIndexes = indexes.groupBy('index_name')
+
+            for ( const key in groupedIndexes ) {
+                if ( !Object.prototype.hasOwnProperty.call(groupedIndexes, key) ) {
+                    continue
+                }
+
+                table.index(key)
+                    .pipe()
+                    .peek(idx => {
+                        collect<{column_name: string}>(groupedIndexes[key])  // eslint-disable-line camelcase
+                            .pluck<string>('column_name')
+                            .each(col => idx.field(col))
+                    })
+                    .when(groupedIndexes[key]?.[0]?.indisprimary, idx => {
+                        idx.primary()
+                    })
+                    .when(groupedIndexes[key]?.[0]?.indisunique, idx => {
+                        idx.unique()
+                    })
+                    .get()
+                    .flagAsExistingInSchema()
+            }
+
+            table.flagAsExistingInSchema()
+        }
+
+        return table
+    }
+
+    /**
+     * Query the database to look up all indexes on a table, by column.
+     * @see https://stackoverflow.com/a/2213199/4971138
+     * @param table
+     * @protected
+     */
+    protected async getIndexes(table: string): Promise<Collection<QueryRow>> {
+        const rawQuery = `
+            select
+                t.relname as table_name,
+                i.relname as index_name,
+                a.attname as column_name,
+                ix.*
+            from pg_class t
+            left join pg_attribute a
+                on a.attrelid = t.oid
+            left join pg_index ix
+                on t.oid = ix.indrelid
+            left join pg_class i
+                on i.oid = ix.indexrelid
+            left join pg_namespace n
+                on n.oid = i.relnamespace
+            where
+                a.attnum = any(ix.indkey)
+                and t.relkind = 'r'
+                and t.relname = '${table}'
+                and n.nspname = '${this.schema}'
+            order by
+                t.relname,
+                i.relname;
+        `
+
+        return (new Builder()).connection(this.connection)
+            .raw(rawQuery)
+            .get()
+            .collect()
+    }
+
+    /**
+     * Query the database to look up all constraints on a table, by column.
+     * @see https://dba.stackexchange.com/a/290854
+     * @param table
+     * @protected
+     */
+    protected async getConstraints(table: string): Promise<Collection<QueryRow>> {
+        const rawQuery = `
+            SELECT * FROM (
+                SELECT
+                    pgc.contype AS constraint_type,
+                    pgc.conname AS constraint_name,
+                    ccu.table_schema AS table_schema,
+                    kcu.table_name AS table_name,
+                    CASE WHEN (pgc.contype = 'f') THEN kcu.COLUMN_NAME ELSE ccu.COLUMN_NAME END AS column_name, 
+                    CASE WHEN (pgc.contype = 'f') THEN ccu.TABLE_NAME ELSE (null) END AS reference_table,
+                    CASE WHEN (pgc.contype = 'f') THEN ccu.COLUMN_NAME ELSE (null) END AS reference_col,
+                    CASE WHEN (pgc.contype = 'p') THEN 'yes' ELSE 'no' END AS auto_inc,
+                    CASE WHEN (pgc.contype = 'p') THEN 'NO' ELSE 'YES' END AS is_nullable,
+                    'integer' AS data_type,
+                    '0' AS numeric_scale,
+                    '32' AS numeric_precision
+                FROM
+                    pg_constraint AS pgc
+                JOIN pg_namespace nsp
+                    ON nsp.oid = pgc.connamespace
+                JOIN pg_class cls
+                    ON pgc.conrelid = cls.oid
+                JOIN information_schema.key_column_usage kcu
+                    ON kcu.constraint_name = pgc.conname
+                LEFT JOIN information_schema.constraint_column_usage ccu
+                    ON pgc.conname = ccu.CONSTRAINT_NAME 
+                    AND nsp.nspname = ccu.CONSTRAINT_SCHEMA
+                WHERE
+                      kcu.table_name = '${table}'
+                UNION
+                    SELECT
+                        NULL AS constraint_type,
+                        NULL AS constraint_name,
+                        table_schema,
+                        table_name,
+                        column_name,
+                        NULL AS refrence_table,
+                        NULL AS refrence_col,
+                        'no' AS auto_inc,
+                        is_nullable,
+                        data_type,
+                        numeric_scale,
+                        numeric_precision
+                    FROM information_schema.columns cols 
+                    WHERE
+                        table_schema = '${this.schema}'
+                        AND table_name = '${table}'
+                ) AS child
+            ORDER BY table_name DESC
+        `
+
+        return (new Builder()).connection(this.connection)
+            .raw(rawQuery)
+            .get()
+            .collect()
+    }
+
+    /**
+     * @see https://dataedo.com/kb/query/postgresql/list-table-check-constraints
+     * @param table
+     * @protected
+     */
+    protected async getCheckConstraints(table: string): Promise<Collection<QueryRow>> {
+        const rawQuery = `
+            SELECT
+                tc.table_schema,
+                tc.table_name,
+                ARRAY_AGG(col.column_name) AS columns,
+                tc.constraint_name,
+                cc.check_clause
+            FROM information_schema.table_constraints tc
+            JOIN information_schema.check_constraints cc
+                ON tc.constraint_schema = cc.constraint_schema
+                    AND tc.constraint_name = cc.constraint_name
+            JOIN pg_namespace nsp
+                ON nsp.nspname = cc.constraint_schema
+            JOIN pg_constraint pgc
+                ON pgc.conname = cc.constraint_name
+                    AND pgc.connamespace = nsp.oid
+                    AND pgc.contype = 'c'
+            JOIN information_schema.columns col
+                ON col.table_schema = tc.table_schema
+                    AND col.table_name = tc.table_name
+                    AND col.ordinal_position = ANY(pgc.conkey)
+            WHERE
+                tc.constraint_schema NOT IN ('pg_catalog', 'information_schema')
+                AND tc.table_schema = '${this.schema}'
+                AND tc.table_name = '${table}'
+            GROUP BY
+                tc.table_schema,
+                tc.table_name,
+                tc.constraint_name,
+                cc.check_clause
+            ORDER BY
+                tc.table_schema,
+                tc.table_name
+        `
+
+        return (new Builder()).connection(this.connection)
+            .raw(rawQuery)
+            .get()
+            .collect()
+    }
+
+    /**
+     * Query the database to look up all columns on a table.
+     * @param table
+     * @protected
+     */
+    protected async getColumns(table: string): Promise<Collection<QueryRow>> {
+        return (new Builder()).connection(this.connection)
+            .select(raw('*'))
+            .from('information_schema.columns')
+            .where('table_schema', '=', this.schema)
+            .where('table_name', '=', table)
+            .get()
+            .collect()
+    }
+}

src/orm/schema/Schema.ts

@@ -1,14 +1,51 @@
 import {Connection} from '../connection/Connection'
 import {Awaitable} from '../../util'
+import {TableBuilder} from './TableBuilder'
 
+/**
+ * Represents a SQL-schema implementation.
+ */
 export abstract class Schema {
     constructor(
+        /** The SQL connection to execute against. */
         protected readonly connection: Connection,
     ) { }
 
+    /**
+     * Resolve true if the schema has a table with the given name.
+     * @param name
+     */
     public abstract hasTable(name: string): Awaitable<boolean>
 
+    /**
+     * Resolve true if the schema table with the given name has a column with the given name.
+     * @param table
+     * @param name
+     */
     public abstract hasColumn(table: string, name: string): Awaitable<boolean>
 
+    /**
+     * Resolve true if the schema table with the given name has all the specified columns.
+     * @param table
+     * @param name
+     */
     public abstract hasColumns(table: string, name: string[]): Awaitable<boolean>
+
+    /**
+     * Get a TableBuilder instance for a table on the schema.
+     * @param table
+     */
+    public abstract table(table: string): Awaitable<TableBuilder>
+
+    /**
+     * Apply the table to the schema.
+     * @param schema
+     */
+    public async commit(schema: TableBuilder): Promise<void> {
+        const query = this.connection
+            .dialect()
+            .renderCommitSchemaTransaction(schema)
+
+        await this.connection.query(query)
+    }
 }

src/orm/schema/TableBuilder.ts

@@ -1,109 +1,770 @@
-import {Pipe} from '../../util'
+import {collect, Maybe, ParameterizedCallback, Pipe} from '../../util'
+import {FieldType} from '../types'
+import {EscapeValue, QuerySafeValue, raw} from '../dialect/SQLDialect'
 
+/**
+ * Base class with shared logic for the various schema
+ * builders (table, column, index).
+ */
 export abstract class SchemaBuilderBase {
+    /**
+     * Whether or not the schema item should be dropped.
+     *  - `exists` - drop if exists
+     * @protected
+     */
     protected shouldDrop: 'yes'|'no'|'exists' = 'no'
 
+    /**
+     * The name the schema item should have if renaming.
+     * @protected
+     */
     protected shouldRenameTo?: string
 
+    /**
+     * If true, apply IF NOT EXISTS syntax.
+     * @protected
+     */
+    protected shouldSkipIfExists = false
+
+    /** True if the schema has been modified since created/loaded. */
+    protected dirty = false
+
+    /** True if this resource exists, in some form, in the schema. */
+    protected existsInSchema = false
+
+    /** If the resource exists in the schema, the unaltered values it has. */
+    public originalFromSchema?: SchemaBuilderBase
+
     constructor(
-        protected readonly name: string,
+        /** The name of the schema item. */
+        public readonly name: string,
     ) { }
 
+    /**
+     * Clone the properties of this resource to a different instance.
+     * @param newBuilder
+     */
+    public cloneTo(newBuilder: SchemaBuilderBase): SchemaBuilderBase {
+        newBuilder.shouldDrop = this.shouldDrop
+        newBuilder.shouldRenameTo = this.shouldRenameTo
+        newBuilder.shouldSkipIfExists = this.shouldSkipIfExists
+        newBuilder.dirty = this.dirty
+        newBuilder.existsInSchema = this.existsInSchema
+        return newBuilder
+    }
+
+    /** True if this resource should be dropped. */
+    public isDropping(): boolean {
+        return this.shouldDrop === 'yes'
+    }
+
+    /** True if this resource should be dropped with IF EXISTS syntax. */
+    public isDroppingIfExists(): boolean {
+        return this.shouldDrop === 'exists'
+    }
+
+    /** True if this resource should be created with IF NOT EXISTS syntax. */
+    public isSkippedIfExisting(): boolean {
+        return this.shouldSkipIfExists
+    }
+
+    /** True if the resource already exists in some form in the schema. */
+    public isExisting(): boolean {
+        return this.existsInSchema
+    }
+
+    /** True if the resource has been modified since created/loaded. */
+    public isDirty(): boolean {
+        return this.dirty
+    }
+
+    /**
+     * Get the name this resource should be renamed to, if it exists.
+     */
+    public getRename(): Maybe<string> {
+        return this.shouldRenameTo
+    }
+
+    /** Mark the resource to be removed. */
     public drop(): this {
+        this.dirty = true
         this.shouldDrop = 'yes'
         return this
     }
 
+    /** Mark the resource to be removed, if it exists. */
     public dropIfExists(): this {
+        this.dirty = true
         this.shouldDrop = 'exists'
         return this
     }
 
+    /**
+     * Rename the resource to a different name.
+     * @param to
+     */
     public rename(to: string): this {
+        this.dirty = true
         this.shouldRenameTo = to
         return this
     }
 
+    /**
+     * Mark the resource to use IF NOT EXISTS syntax.
+     */
+    public ifNotExists(): this {
+        this.shouldSkipIfExists = true
+        return this
+    }
+
+    /**
+     * Used internally.
+     * Mark that the resource exists in the schema in some form,
+     * and reset the `dirty` flag.
+     */
+    public flagAsExistingInSchema(): this {
+        this.existsInSchema = true
+        this.dirty = false
+        this.originalFromSchema = this.cloneTo(this.cloneInstance())
+        return this
+    }
+
+    /** Get a Pipe containing this instance. */
     pipe(): Pipe<this> {
         return Pipe.wrap<this>(this)
     }
+
+    /**
+     * Get a new instance of the concrete implementation of this class.
+     * @protected
+     */
+    protected abstract cloneInstance(): this
 }
 
+/**
+ * Builder to specify the schema of a table column.
+ */
 export class ColumnBuilder extends SchemaBuilderBase {
+    /** The data type of the column. */
+    protected targetType?: FieldType
 
-}
+    /** True if the column should allow NULL values. */
+    protected shouldBeNullable = false
 
-export class IndexBuilder extends SchemaBuilderBase {
+    /** The default value of the column, if one should exist. */
+    protected defaultValue?: EscapeValue
 
-    protected fields: Set<string> = new Set<string>()
-
-    protected removedFields: Set<string> = new Set<string>()
-
-    protected shouldBeUnique = false
+    /** The data length of this column, if set */
+    protected targetLength?: number
 
+    /** True if this is a primary key constraint. */
     protected shouldBePrimary = false
 
-    protected field(name: string): this {
+    /** True if this column should contain distinct values. */
+    protected shouldBeUnique = false
+
+    public originalFromSchema?: ColumnBuilder
+
+    constructor(
+        name: string,
+
+        /** The table this column belongs to. */
+        public readonly parent: TableBuilder,
+    ) {
+        super(name)
+    }
+
+    public cloneTo(newBuilder: ColumnBuilder): ColumnBuilder {
+        super.cloneTo(newBuilder)
+        newBuilder.targetType = this.targetType
+        newBuilder.shouldBeNullable = this.shouldBeNullable
+        newBuilder.defaultValue = this.defaultValue
+        newBuilder.targetLength = this.targetLength
+        newBuilder.shouldBePrimary = this.shouldBePrimary
+        newBuilder.shouldBeUnique = this.shouldBeUnique
+        return newBuilder
+    }
+
+    /** Get the field type of the column, if it exists. */
+    public getType(): Maybe<FieldType> {
+        return this.targetType
+    }
+
+    /** Get the data-type length of the column, if it exists. */
+    public getLength(): Maybe<number> {
+        return this.targetLength
+    }
+
+    /** Get the default value of the column, if it exists. */
+    public getDefaultValue(): Maybe<EscapeValue> {
+        return this.defaultValue
+    }
+
+    /** True if the column should allow NULL values. */
+    public isNullable(): boolean {
+        return this.shouldBeNullable
+    }
+
+    /** True if the column is a primary key. */
+    public isPrimary(): boolean {
+        return this.shouldBePrimary
+    }
+
+    /** True if the column should require unique values. */
+    public isUnique(): boolean {
+        return this.shouldBeUnique
+    }
+
+    /**
+     * Specify the data type of the column.
+     * @param type
+     */
+    public type(type: FieldType): this {
+        if ( this.targetType === type ) {
+            return this
+        }
+
+        this.dirty = true
+        this.targetType = type
+        return this
+    }
+
+    /**
+     * Make the column nullable.
+     */
+    public nullable(): this {
+        if ( this.shouldBeNullable ) {
+            return this
+        }
+
+        this.dirty = true
+        this.shouldBeNullable = true
+        return this
+    }
+
+    /**
+     * Make the column non-nullable.
+     */
+    public required(): this {
+        if ( !this.shouldBeNullable ) {
+            return this
+        }
+
+        this.dirty = true
+        this.shouldBeNullable = false
+        return this
+    }
+
+    /**
+     * Specify the default value of the column.
+     * @param value
+     */
+    public default(value: EscapeValue): this {
+        if ( this.defaultValue === value ) {
+            return this
+        }
+
+        this.dirty = true
+        this.defaultValue = value
+        return this
+    }
+
+    /**
+     * Set the length of this column's data type.
+     * @param value
+     */
+    public length(value: number): this {
+        if ( this.targetLength === value ) {
+            return this
+        }
+
+        this.dirty = true
+        this.targetLength = value
+        return this
+    }
+
+    /**
+     * Make this a primary-key column.
+     */
+    primary(): this {
+        if ( this.shouldBePrimary ) {
+            return this
+        }
+
+        this.dirty = true
+        this.shouldBePrimary = true
+        return this
+    }
+
+    /**
+     * Make this column require distinct values.
+     */
+    unique(): this {
+        if ( this.shouldBeUnique ) {
+            return this
+        }
+
+        this.dirty = true
+        this.shouldBeUnique = true
+        return this
+    }
+
+    protected cloneInstance(): this {
+        return new ColumnBuilder(this.name, this.parent) as this
+    }
+}
+
+/** Valid constraint types. */
+export enum ConstraintType {
+    Unique = 'un',
+    Check = 'ck',
+}
+
+/**
+ * Builder to specify the schema of a table constraint.
+ */
+export class ConstraintBuilder extends SchemaBuilderBase {
+    public originalFromSchema?: ConstraintBuilder
+
+    /** The fields included in this constraint. */
+    protected fields: Set<string> = new Set<string>()
+
+    /** The type of this constraint. */
+    protected constraintType: ConstraintType = ConstraintType.Unique
+
+    /** The expression defining this constraint, if applicable. */
+    protected constraintExpression?: QuerySafeValue
+
+    constructor(
+        name: string,
+
+        /** The table this constraint belongs to. */
+        public readonly parent: TableBuilder,
+    ) {
+        super(name)
+    }
+
+    /** Get the type of this constraint. */
+    public getType(): ConstraintType {
+        return this.constraintType
+    }
+
+    /** Get the fields included in this constraint. */
+    public getFields(): string[] {
+        return [...this.fields]
+    }
+
+    /** Get the expression used to evaluate this constraint, if it exists. */
+    public getExpression(): Maybe<QuerySafeValue> {
+        return this.constraintExpression
+    }
+
+    public cloneTo(newBuilder: ConstraintBuilder): ConstraintBuilder {
+        super.cloneTo(newBuilder)
+        newBuilder.fields = new Set<string>([...this.fields])
+        newBuilder.constraintType = this.constraintType
+        return newBuilder
+    }
+
+    protected cloneInstance(): this {
+        return new ConstraintBuilder(this.name, this.parent) as this
+    }
+
+    /** Add a field to this constraint. */
+    public field(name: string): this {
+        if ( this.fields.has(name) ) {
+            return this
+        }
+
+        this.dirty = true
         this.fields.add(name)
         return this
     }
 
+    /** Remove a field from this constraint. */
+    public removeField(name: string): this {
+        if ( !this.fields.has(name) ) {
+            return this
+        }
+
+        this.dirty = true
+        this.fields.delete(name)
+        return this
+    }
+
+    /** Specify the type of this constraint. */
+    public type(type: ConstraintType): this {
+        if ( this.constraintType === type ) {
+            return this
+        }
+
+        this.dirty = true
+        this.constraintType = type
+        return this
+    }
+
+    /** Specify the expression used to evaluate this constraint, if applicable. */
+    public expression(sql: string | QuerySafeValue): this {
+        if ( String(this.constraintExpression) === String(sql) ) {
+            return this
+        }
+
+        this.dirty = true
+
+        if ( sql instanceof QuerySafeValue ) {
+            this.constraintExpression = sql
+        }
+
+        this.constraintExpression = raw(sql)
+        return this
+    }
+}
+
+/**
+ * Builder to specify the schema of a table index.
+ */
+export class IndexBuilder extends SchemaBuilderBase {
+    /** The fields included in the index. */
+    protected fields: Set<string> = new Set<string>()
+
+    /** Fields to remove from the index. */
+    protected removedFields: Set<string> = new Set<string>()
+
+    /** True if this is a unique index. */
+    protected shouldBeUnique = false
+
+    /** True if this is a primary key index. */
+    protected shouldBePrimary = false
+
+    public originalFromSchema?: IndexBuilder
+
+    constructor(
+        name: string,
+
+        /** The table this index belongs to. */
+        public readonly parent: TableBuilder,
+    ) {
+        super(name)
+    }
+
+    public cloneTo(newBuilder: IndexBuilder): IndexBuilder {
+        super.cloneTo(newBuilder)
+        newBuilder.fields = new Set<string>([...this.fields])
+        newBuilder.removedFields = new Set<string>([...this.removedFields])
+        newBuilder.shouldBeUnique = this.shouldBeUnique
+        newBuilder.shouldBePrimary = this.shouldBePrimary
+        return newBuilder
+    }
+
+    /** Get the fields in this index. */
+    public getFields(): string[] {
+        return [...this.fields]
+    }
+
+    /** True if this index is a unique index. */
+    public isUnique(): boolean {
+        return this.shouldBeUnique
+    }
+
+    /** True if this index is the primary key index. */
+    public isPrimary(): boolean {
+        return this.shouldBePrimary
+    }
+
+    /**
+     * Add the given field to this index.
+     * @param name
+     */
+    public field(name: string): this {
+        if ( this.fields.has(name) ) {
+            return this
+        }
+
+        this.dirty = true
+        this.fields.add(name)
+        return this
+    }
+
+    /**
+     * Remove the given field from this index.
+     * @param name
+     * @protected
+     */
     protected removeField(name: string): this {
+        if ( !this.fields.has(name) ) {
+            return this
+        }
+
+        this.dirty = true
         this.removedFields.add(name)
         this.fields.delete(name)
         return this
     }
 
+    /**
+     * Make this a primary-key index.
+     */
     primary(): this {
+        if ( this.shouldBePrimary ) {
+            return this
+        }
+
+        this.dirty = true
         this.shouldBePrimary = true
         return this
     }
 
+    /**
+     * Make this a unique index.
+     */
     unique(): this {
+        if ( this.shouldBeUnique ) {
+            return this
+        }
+
+        this.dirty = true
         this.shouldBeUnique = true
         return this
     }
+
+    protected cloneInstance(): this {
+        return new IndexBuilder(this.name, this.parent) as this
+    }
 }
 
+/**
+ * Builder to specify the schema of a table.
+ */
 export class TableBuilder extends SchemaBuilderBase {
 
+    /**
+     * Mapping of column name to column schemata.
+     * @protected
+     */
     protected columns: {[key: string]: ColumnBuilder} = {}
 
+    /**
+     * Mapping of index name to index schemata.
+     * @protected
+     */
     protected indexes: {[key: string]: IndexBuilder} = {}
 
+    /**
+     * Mapping of constraint name to constraint schemata.
+     * @protected
+     */
+    protected constraints: {[key: string]: ConstraintBuilder} = {}
+
+    public originalFromSchema?: TableBuilder
+
+    public cloneTo(newBuilder: TableBuilder): TableBuilder {
+        super.cloneTo(newBuilder)
+        newBuilder.columns = {...this.columns}
+        newBuilder.indexes = {...this.indexes}
+        newBuilder.constraints = {...this.constraints}
+        return newBuilder
+    }
+
+    /**
+     * Get the columns defined on this table.
+     */
+    public getColumns(): {[key: string]: ColumnBuilder} {
+        return {
+            ...this.columns,
+        }
+    }
+
+    /**
+     * Get the indices defined on this table.
+     */
+    public getIndexes(): {[key: string]: IndexBuilder} {
+        return {
+            ...this.indexes,
+        }
+    }
+
+    /**
+     * Get the constraints defined on this table.
+     */
+    public getConstraints(): {[key: string]: ConstraintBuilder} {
+        return {
+            ...this.constraints,
+        }
+    }
+
+    /**
+     * Mark a column to be dropped.
+     * @param name
+     */
     public dropColumn(name: string): this {
+        this.dirty = true
         this.column(name).drop()
         return this
     }
 
+    /**
+     * Mark a column to be renamed.
+     * @param from
+     * @param to
+     */
     public renameColumn(from: string, to: string): this {
         this.column(from).rename(to)
         return this
     }
 
+    /**
+     * Mark an index to be dropped.
+     * @param name
+     */
     public dropIndex(name: string): this {
+        this.dirty = true
         this.index(name).drop()
         return this
     }
 
+    /**
+     * Mark an index to be renamed.
+     * @param from
+     * @param to
+     */
     public renameIndex(from: string, to: string): this {
         this.index(from).rename(to)
         return this
     }
 
-    public column(name: string) {
+    /**
+     * Add a column to this table.
+     * @param name
+     * @param callback
+     */
+    public column(name: string, callback?: ParameterizedCallback<ColumnBuilder>): ColumnBuilder {
         if ( !this.columns[name] ) {
-            this.columns[name] = new ColumnBuilder(name)
+            this.dirty = true
+            this.columns[name] = new ColumnBuilder(name, this)
+        }
+
+        if ( callback ) {
+            callback(this.columns[name])
         }
 
         return this.columns[name]
     }
 
-    public index(name: string) {
+    /**
+     * Add an index to this table.
+     * @param name
+     * @param callback
+     */
+    public index(name: string, callback?: ParameterizedCallback<IndexBuilder>): IndexBuilder {
         if ( !this.indexes[name] ) {
-            this.indexes[name] = new IndexBuilder(name)
+            this.dirty = true
+            this.indexes[name] = new IndexBuilder(name, this)
+        }
+
+        if ( callback ) {
+            callback(this.indexes[name])
         }
 
         return this.indexes[name]
     }
+
+    /**
+     * Add a constraint to this table.
+     * @param name
+     * @param callback
+     */
+    public constraint(name: string, callback?: ParameterizedCallback<ConstraintBuilder>): ConstraintBuilder {
+        if ( !this.constraints[name] ) {
+            this.dirty = true
+            this.constraints[name] = new ConstraintBuilder(name, this)
+        }
+
+        if ( callback ) {
+            callback(this.constraints[name])
+        }
+
+        return this.constraints[name]
+    }
+
+    /**
+     * Generate a programmatically-incrementing constraint name.
+     * @param suffix
+     * @protected
+     */
+    protected getNextAvailableConstraintName(suffix: ConstraintType): string {
+        let current = 1
+        let name = `${this.name}_${current}_${suffix}`
+
+        while ( this.constraints[name] ) {
+            current += 1
+            name = `${this.name}_${current}_${suffix}`
+        }
+
+        return name
+    }
+
+    /**
+     * Add a new check constraint with the given expression.
+     * @param expression
+     */
+    public check(expression: string | QuerySafeValue): this {
+        const name = this.getNextAvailableConstraintName(ConstraintType.Check)
+        this.constraint(name)
+            .type(ConstraintType.Check)
+            .expression(expression)
+
+        return this
+    }
+
+    /**
+     * Add a new unique constraint for the given fields.
+     * @param fields
+     */
+    public unique(...fields: string[]): this {
+        // First, check if an existing constraint exists with these fields
+        for ( const key in this.constraints ) {
+            if ( !Object.prototype.hasOwnProperty.call(this.constraints, key) ) {
+                continue
+            }
+
+            if ( this.constraints[key].getType() !== ConstraintType.Unique ) {
+                continue
+            }
+
+            const existingFields = collect<string>(this.constraints[key].getFields())
+            const intersection = existingFields.intersect(fields)
+
+            if ( existingFields.length === fields.length && intersection.length === fields.length ) {
+                return this
+            }
+        }
+
+        // If an existing constraint can't satisfy this, create a new one
+        const name = this.getNextAvailableConstraintName(ConstraintType.Unique)
+        this.constraint(name)
+            .type(ConstraintType.Unique)
+            .pipe()
+            .peek(constraint => {
+                fields.forEach(field => constraint.field(field))
+            })
+
+        return this
+    }
+
+    /**
+     * Add a primary key (column & index) to this table.
+     * @param name
+     * @param type
+     */
+    public primaryKey(name: string, type: FieldType = FieldType.serial): ColumnBuilder {
+        this.dirty = true
+
+        return this.column(name)
+            .type(type)
+            .primary()
+    }
+
+    protected cloneInstance(): this {
+        return new TableBuilder(this.name) as this
+    }
 }

src/orm/services/Database.ts

@@ -1,10 +1,11 @@
-import {Inject, Singleton} from '../../di'
+import {Container, Inject, Singleton} from '../../di'
 import {DatabaseService} from '../DatabaseService'
 import {PostgresConnection} from '../connection/PostgresConnection'
 import {ErrorWithContext} from '../../util'
 import {Unit} from '../../lifecycle/Unit'
 import {Config} from '../../service/Config'
 import {Logging} from '../../service/Logging'
+import {MigratorFactory} from '../migrations/MigratorFactory'
 
 /**
  * Application unit responsible for loading and creating database connections from config.
@@ -12,13 +13,16 @@ import {Logging} from '../../service/Logging'
 @Singleton()
 export class Database extends Unit {
     @Inject()
-    protected readonly config!: Config;
+    protected readonly config!: Config
 
     @Inject()
-    protected readonly dbService!: DatabaseService;
+    protected readonly dbService!: DatabaseService
 
     @Inject()
-    protected readonly logging!: Logging;
+    protected readonly logging!: Logging
+
+    @Inject('injector')
+    protected readonly injector!: Container
 
     /**
      * Load the `database.connections` config and register Connection instances for each config.
@@ -28,6 +32,9 @@ export class Database extends Unit {
         const connections = this.config.get('database.connections')
         const promises = []
 
+        // Register the migrator factory
+        this.injector.registerFactory(this.injector.make(MigratorFactory))
+
         for ( const key in connections ) {
             if ( !Object.prototype.hasOwnProperty.call(connections, key) ) {
                 continue

src/orm/services/Migrations.ts

@@ -0,0 +1,98 @@
+import {Inject, Singleton} from '../../di'
+import {CanonicalInstantiable} from '../../service/CanonicalInstantiable'
+import {Migration} from '../migrations/Migration'
+import {CanonicalDefinition, CanonicalResolver} from '../../service/Canonical'
+import {Migrator} from '../migrations/Migrator'
+import {UniversalPath} from '../../util'
+import {lib} from '../../lib'
+import {CommandLine} from '../../cli'
+import {MigrateDirective} from '../directive/MigrateDirective'
+import {RollbackDirective} from '../directive/RollbackDirective'
+import {CreateMigrationDirective} from '../directive/CreateMigrationDirective'
+
+/**
+ * Service unit that loads and instantiates migration classes.
+ */
+@Singleton()
+export class Migrations extends CanonicalInstantiable<Migration> {
+    @Inject()
+    protected readonly migrator!: Migrator
+
+    @Inject()
+    protected readonly cli!: CommandLine
+
+    protected appPath = ['migrations']
+
+    protected canonicalItem = 'migration'
+
+    protected suffix = '.migration.js'
+
+    async up(): Promise<void> {
+        if ( await this.path.exists() ) {
+            await super.up()
+        } else {
+            this.logging.debug(`Base migration path does not exist, or has no files: ${this.path}`)
+        }
+
+        // Register the migrations for @extollo/lib
+        const basePath = lib().concat('migrations')
+        const resolver = await this.buildMigrationNamespaceResolver('@extollo', basePath)
+        this.registerNamespace('@extollo', resolver)
+
+        // Register the migrate CLI directives
+        this.cli.registerDirective(MigrateDirective)
+        this.cli.registerDirective(RollbackDirective)
+        this.cli.registerDirective(CreateMigrationDirective)
+    }
+
+    async initCanonicalItem(definition: CanonicalDefinition): Promise<Migration> {
+        const instance = await super.initCanonicalItem(definition)
+
+        if ( !(instance instanceof Migration) ) {
+            throw new TypeError(`Invalid migration: ${definition.originalName}. Migrations must extend from @extollo/lib.Migration.`)
+        }
+
+        instance.setMigrationIdentifier(definition.canonicalName)
+
+        return instance
+    }
+
+    /**
+     * Creates a CanonicalResolver for a directory that contains migration files.
+     *
+     * @example
+     * ```ts
+     * const path = universalPath('path', 'to', 'migrations', 'folder')
+     * const namespace = '@mypackage'
+     *
+     * const resolver = await migrations.buildMigrationNamespaceResolver(namespace, path)
+     * migrations.registerNamespace(namespace, resolver)
+     * ```
+     * @param name
+     * @param basePath
+     */
+    public async buildMigrationNamespaceResolver(name: string, basePath: UniversalPath): Promise<CanonicalResolver<Migration>> {
+        if ( !name.startsWith('@') ) {
+            name = `@${name}`
+        }
+
+        const namespace: {[key: string]: Migration} = {}
+
+        for await ( const entry of basePath.walk() ) {
+            if ( !entry.endsWith(this.suffix) ) {
+                this.logging.debug(`buildMigrationNamespaceResolver - Skipping file with invalid suffix: ${entry}`)
+                continue
+            }
+
+            const definition = await this.buildCanonicalDefinition(entry, basePath)
+            this.logging.verbose(`buildMigrationNamespaceResolver - Discovered canonical ${this.canonicalItem} "${definition.canonicalName}" from ${entry}`)
+            namespace[definition.canonicalName] = await this.initCanonicalItem(definition)
+            namespace[definition.canonicalName].setMigrationIdentifier(`${name}:${namespace[definition.canonicalName].identifier}`)
+        }
+
+        return {
+            get: (key: string) => namespace[key],
+            all: () => Object.keys(namespace),
+        }
+    }
+}

src/orm/template/migration.ts

@@ -0,0 +1,40 @@
+import {Template} from '../../cli'
+
+/**
+ * Template for creating new migration classes in app/migrations.
+ */
+const templateMigration: Template = {
+    name: 'migration',
+    fileSuffix: '.migration.ts',
+    baseAppPath: ['migrations'],
+    description: 'Create a new class that applies a one-time migration',
+    render: (name: string) => {
+        return `import {Injectable, Migration} from '@extollo/lib'
+
+/**
+ * ${name}
+ * ----------------------------------
+ * Put some description here.
+ */
+@Injectable()
+export default class ${name} extends Migration {
+
+    /**
+     * Apply the migration.
+     */
+    async up(): Promise<void> {
+
+    }
+
+    /**
+     * Undo the migration.
+     */
+    async down(): Promise<void> {
+
+    }
+}
+`
+    },
+}
+
+export { templateMigration }

src/orm/types.ts

@@ -152,3 +152,56 @@ export enum FieldType {
     other = 'other',
 }
 
+/**
+ * Given a FieldType, get the inverse (that is, the code-form name).
+ * @example
+ * ```ts
+ * console.log(FieldType.varchar)  // => "character varying"
+ * console.log(inverseFieldType(FieldType.varchar))  // => "varchar"
+ * console.log(inverseFieldType('character varying'))  // => "varchar"
+ * ```
+ * @param type
+ */
+export function inverseFieldType(type: FieldType): string {
+    return ({
+        bigint: 'bigint',
+        bigserial: 'bigserial',
+        bit: 'bit',
+        'bit varying': 'varbit',
+        boolean: 'boolean',
+        box: 'box',
+        bytea: 'bytea',
+        character: 'character',
+        char: 'character',
+        'character varying': 'varchar',
+        cidr: 'cidr',
+        circle: 'circle',
+        date: 'date',
+        'double precision': 'float8',
+        inet: 'inet',
+        integer: 'integer',
+        interval: 'interval',
+        json: 'json',
+        line: 'line',
+        lseg: 'lseg',
+        macaddr: 'macaddr',
+        money: 'money',
+        numeric: 'numeric',
+        path: 'path',
+        point: 'point',
+        polygon: 'polygon',
+        real: 'real',
+        smallint: 'smallint',
+        smallserial: 'smallserial',
+        serial: 'serial',
+        text: 'text',
+        time: 'time',
+        timestamp: 'timestamp',
+        tsquery: 'tsquery',
+        tsvector: 'tsvector',
+        txidSnapshot: 'txidSnapshot',
+        uuid: 'uuid',
+        xml: 'xml',
+        other: 'other',
+    })[type]
+}

src/service/Canonical.ts

@@ -20,7 +20,17 @@ export interface CanonicalDefinition {
 /**
  * Type alias for a function that resolves a canonical name to a canonical item, if one exists.
  */
-export type CanonicalResolver<T> = (key: string) => T | undefined
+export type CanonicalResolverFunction<T> = (key: string) => T | undefined
+
+/**
+ * Interface for a canonical resolver that provides additional information.
+ */
+export interface ComplexCanonicalResolver<T> {
+    get: CanonicalResolverFunction<T>,
+    all: () => string[],
+}
+
+export type CanonicalResolver<T> = CanonicalResolverFunction<T> | ComplexCanonicalResolver<T>
 
 /**
  * Base type for a canonical name reference.
@@ -105,10 +115,33 @@ export abstract class Canonical<T> extends Unit {
     /**
      * Return an array of all loaded canonical names.
      */
-    public all(): string[] {
+    public all(namespace?: string): string[] {
+        if ( namespace ) {
+            const resolver = this.loadedNamespaces[namespace]
+            if ( !resolver ) {
+                throw new ErrorWithContext(`Unable to find namespace for ${this.canonicalItem}: ${namespace}`, {
+                    canonicalItem: this.canonicalItem,
+                    namespace,
+                })
+            }
+
+            if ( typeof resolver === 'function' ) {
+                return []
+            } else {
+                return resolver.all()
+            }
+        }
+
         return Object.keys(this.loadedItems)
     }
 
+    /**
+     * Return an array of all loaded canonical namespaces.
+     */
+    public namespaces(): string[] {
+        return Object.keys(this.loadedNamespaces)
+    }
+
     /**
      * Get a Universal path to the base directory where this unit loads its canonical files from.
      */
@@ -127,7 +160,8 @@ export abstract class Canonical<T> extends Unit {
             const [namespace, ...rest] = key.split(':')
             key = rest.join(':')
 
-            if ( !this.loadedNamespaces[namespace] ) {
+            const resolver = this.loadedNamespaces[namespace]
+            if ( !resolver ) {
                 throw new ErrorWithContext(`Unable to find namespace for ${this.canonicalItem}: ${namespace}`, {
                     canonicalItem: this.canonicalItem,
                     namespace,
@@ -135,7 +169,11 @@ export abstract class Canonical<T> extends Unit {
                 })
             }
 
-            return this.loadedNamespaces[namespace](key)
+            if ( typeof resolver === 'function' ) {
+                return resolver(key)
+            } else {
+                return resolver.get(key)
+            }
         }
 
         return this.loadedItems[key]
@@ -208,10 +246,11 @@ export abstract class Canonical<T> extends Unit {
     /**
      * Given the path to a file in the canonical items directory, create a CanonicalDefinition record from that file.
      * @param filePath
+     * @param basePath
      * @protected
      */
-    protected async buildCanonicalDefinition(filePath: string): Promise<CanonicalDefinition> {
-        const originalName = filePath.replace(this.path.toLocal, '').substr(1)
+    protected async buildCanonicalDefinition(filePath: string, basePath?: UniversalPath): Promise<CanonicalDefinition> {
+        const originalName = filePath.replace((basePath || this.path).toLocal, '').substr(1)
         const pathRegex = new RegExp(nodePath.sep, 'g')
         const canonicalName = originalName.replace(pathRegex, ':')
             .split('')

src/service/CanonicalInstantiable.ts

@@ -3,7 +3,7 @@
  * @extends Error
  */
 import {Canonical, CanonicalDefinition} from './Canonical'
-import {Instantiable, isInstantiable} from '../di'
+import {isInstantiable} from '../di'
 
 /**
  * Error thrown when the export of a canonical file is determined to be invalid.
@@ -17,8 +17,8 @@ export class InvalidCanonicalExportError extends Error {
 /**
  * Variant of the Canonical unit whose files export classes which are instantiated using the global container.
  */
-export class CanonicalInstantiable<T> extends Canonical<Instantiable<T>> {
-    public async initCanonicalItem(definition: CanonicalDefinition): Promise<Instantiable<T>> {
+export class CanonicalInstantiable<T> extends Canonical<T> {
+    public async initCanonicalItem(definition: CanonicalDefinition): Promise<T> {
         if ( isInstantiable(definition.imported.default) ) {
             return this.app().make(definition.imported.default)
         }

src/util/collection/AsyncCollection.ts

@@ -7,6 +7,7 @@ import {
 } from './Collection'
 import {Iterable, StopIteration} from './Iterable'
 import {applyWhere, WhereOperator} from './where'
+import {AsyncPipe, Pipe} from '../support/Pipe'
 type AsyncCollectionComparable<T> = CollectionItem<T>[] | Collection<T> | AsyncCollection<T>
 type AsyncKeyFunction<T, T2> = (item: CollectionItem<T>, index: number) => CollectionItem<T2> | Promise<CollectionItem<T2>>
 type AsyncCollectionFunction<T, T2> = (items: AsyncCollection<T>) => T2
@@ -318,6 +319,24 @@ export class AsyncCollection<T> {
         return new Collection<T2>(newItems)
     }
 
+    /**
+     * Create a new collection by mapping the items in this collection using the given function,
+     * excluding any for which the function resolves undefined.
+     * @param func
+     */
+    async partialMap<T2>(func: AsyncKeyFunction<T, T2 | undefined>): Promise<Collection<NonNullable<T2>>> {
+        const newItems: CollectionItem<NonNullable<T2>>[] = []
+
+        await this.each(async (item, index) => {
+            const result = await func(item, index)
+            if ( typeof result !== 'undefined' ) {
+                newItems.push(result as NonNullable<T2>)
+            }
+        })
+
+        return new Collection<NonNullable<T2>>(newItems)
+    }
+
     /**
      * Returns true if the given operator returns true for every item in the collection.
      * @param {AsyncKeyFunction} func
@@ -783,10 +802,24 @@ export class AsyncCollection<T> {
      * Return the value of the function, passing this collection to it.
      * @param {AsyncCollectionFunction} func
      */
-    pipe<T2>(func: AsyncCollectionFunction<T, T2>): any {
+    pipeTo<T2>(func: AsyncCollectionFunction<T, T2>): any {
         return func(this)
     }
 
+    /**
+     * Return a new Pipe of this collection.
+     */
+    pipe(): Pipe<AsyncCollection<T>> {
+        return Pipe.wrap(this)
+    }
+
+    /**
+     * Return a new AsyncPipe of this collection.
+     */
+    asyncPipe(): AsyncPipe<AsyncCollection<T>> {
+        return AsyncPipe.wrap(this)
+    }
+
     /* async pop(): Promise<MaybeCollectionItem<T>> {
         const nextItem = await this.storedItems.next()
         if ( !nextItem.done ) {

src/util/collection/Collection.ts

@@ -1,11 +1,11 @@
-import {Pipe} from '../support/Pipe'
+import {AsyncPipe, Pipe} from '../support/Pipe'
 
 type CollectionItem<T> = T
 type MaybeCollectionItem<T> = CollectionItem<T> | undefined
 type KeyFunction<T, T2> = (item: CollectionItem<T>, index: number) => CollectionItem<T2>
 type KeyReducerFunction<T, T2> = (current: any, item: CollectionItem<T>, index: number) => T2
 type CollectionFunction<T, T2> = (items: Collection<T>) => T2
-type KeyOperator<T, T2> = string | KeyFunction<T, T2>
+type KeyOperator<T, T2> = keyof T | KeyFunction<T, T2>
 type AssociatedCollectionItem<T2, T> = { key: T2, item: CollectionItem<T> }
 type CollectionComparable<T> = CollectionItem<T>[] | Collection<T>
 type DeterminesEquality<T> = (item: CollectionItem<T>, other: any) => boolean
@@ -313,6 +313,24 @@ class Collection<T> {
         return new Collection<T2>(newItems)
     }
 
+    /**
+     * Create a new collection by mapping the items in this collection using the given function,
+     * excluding any for which the function returns undefined.
+     * @param func
+     */
+    partialMap<T2>(func: KeyFunction<T, T2 | undefined>): Collection<NonNullable<T2>> {
+        const newItems: CollectionItem<NonNullable<T2>>[] = []
+
+        this.each(((item, index) => {
+            const result = func(item, index)
+            if ( typeof result !== 'undefined' ) {
+                newItems.push(result as NonNullable<T2>)
+            }
+        }))
+
+        return new Collection<NonNullable<T2>>(newItems)
+    }
+
     /**
      * Convert this collection to an object keyed by the given field.
      *
@@ -354,10 +372,10 @@ class Collection<T> {
         this.allAssociated(key).forEach(assoc => {
             i += 1
 
-            if ( typeof value === 'string' ) {
-                obj[assoc.key] = (assoc.item as any)[value]
-            } else {
+            if ( typeof value === 'function' ) {
                 obj[assoc.key] = value(assoc.item, i)
+            } else {
+                obj[assoc.key] = (assoc.item[value] as any) as T2
             }
         })
 
@@ -805,6 +823,13 @@ class Collection<T> {
         return Pipe.wrap(this)
     }
 
+    /**
+     * Return a new AsyncPipe of this collection.
+     */
+    asyncPipe(): AsyncPipe<Collection<T>> {
+        return AsyncPipe.wrap(this)
+    }
+
     /**
      * Remove the last item from this collection.
      */

src/util/index.ts

@@ -13,6 +13,7 @@ export * from './error/ErrorWithContext'
 
 export * from './logging/Logger'
 export * from './logging/StandardLogger'
+export * from './logging/FileLogger'
 export * from './logging/types'
 
 export * from './support/BehaviorSubject'

src/util/logging/FileLogger.ts

@@ -0,0 +1,44 @@
+import {Logger} from './Logger'
+import {LogMessage} from './types'
+import {Injectable} from '../../di'
+import {universalPath} from '../support/path'
+import {appPath, env} from '../../lifecycle/Application'
+import {Writable} from 'stream'
+
+/**
+ * A Logger implementation that writes to a UniversalPath.
+ */
+@Injectable()
+export class FileLogger extends Logger {
+    private resolvedPath?: Writable
+
+    /**
+     * Get the re-usable write stream to the log file.
+     * @protected
+     */
+    protected async getWriteStream(): Promise<Writable> {
+        if ( !this.resolvedPath ) {
+            let basePath = env('EXTOLLO_LOGGING_FILE')
+            if ( basePath && !Array.isArray(basePath) ) {
+                basePath = [basePath]
+            }
+
+            const resolvedPath = basePath ? universalPath(...basePath) : appPath('..', '..', 'extollo.log')
+
+            if ( !(await resolvedPath.exists()) ) {
+                await resolvedPath.concat('..').mkdir()
+                await resolvedPath.write('')
+            }
+
+            this.resolvedPath = await resolvedPath.writeStream()
+        }
+
+        return this.resolvedPath
+    }
+
+    public async write(message: LogMessage): Promise<void> {
+        const text = `${message.level} ${this.formatDate(message.date)} (${message.callerName || 'Unknown'}) ${message.output}`
+        const stream = await this.getWriteStream()
+        stream.write(text + '\n')
+    }
+}

src/util/logging/Logger.ts

@@ -1,5 +1,6 @@
 import {LoggingLevel, LogMessage} from './types'
 import * as color from 'colors/safe'
+import {Awaitable} from '../support/types'
 
 /**
  * Base class for an application logger.
@@ -10,7 +11,7 @@ export abstract class Logger {
      * @param {LogMessage} message
      * @return Promise<void>
      */
-    public abstract write(message: LogMessage): Promise<void> | void;
+    public abstract write(message: LogMessage): Awaitable<void>;
 
     /**
      * Format the date object to the string output format.

src/util/support/Pipe.ts

@@ -1,12 +1,14 @@
 /**
  * A closure that maps a given pipe item to a different type.
  */
+import {Awaitable} from './types'
+
 export type PipeOperator<T, T2> = (subject: T) => T2
 
 /**
  * A closure that maps a given pipe item to an item of the same type.
  */
-export type ReflexivePipeOperator<T> = (subject: T) => T
+export type ReflexivePipeOperator<T> = (subject: T) => T|void
 
 /**
  * A condition or condition-resolving function for pipe methods.
@@ -77,6 +79,15 @@ export class Pipe<T> {
         return new Pipe(op(this.subject))
     }
 
+    /**
+     * Like tap, but always returns the original pipe.
+     * @param op
+     */
+    peek<T2>(op: PipeOperator<T, T2>): this {
+        op(this.subject)
+        return this
+    }
+
     /**
      * If `check` is truthy, apply the given operator to the item in the pipe and return the result.
      * Otherwise, just return the current pipe unchanged.
@@ -86,7 +97,7 @@ export class Pipe<T> {
      */
     when(check: PipeCondition<T>, op: ReflexivePipeOperator<T>): Pipe<T> {
         if ( (typeof check === 'function' && check(this.subject)) || check ) {
-            return Pipe.wrap(op(this.subject))
+            Pipe.wrap(op(this.subject))
         }
 
         return this
@@ -104,7 +115,8 @@ export class Pipe<T> {
             return this
         }
 
-        return Pipe.wrap(op(this.subject))
+        Pipe.wrap(op(this.subject))
+        return this
     }
 
     /**
@@ -127,4 +139,134 @@ export class Pipe<T> {
     get(): T {
         return this.subject
     }
+
+    /**
+     * Get an AsyncPipe with the current item in the pipe.
+     */
+    async(): AsyncPipe<T> {
+        return AsyncPipe.wrap<T>(this.subject)
+    }
+}
+
+/**
+ * A subject function that yields the value in the AsyncPipe.
+ */
+export type AsyncPipeResolver<T> = () => Awaitable<T>
+
+/**
+ * A closure that maps a given pipe item to a different type.
+ */
+export type AsyncPipeOperator<T, T2> = (subject: T) => Awaitable<T2>
+
+/**
+ * A closure that maps a given pipe item to an item of the same type.
+ */
+export type ReflexiveAsyncPipeOperator<T> = (subject: T) => Awaitable<T|void>
+
+/**
+ * A condition or condition-resolving function for pipe methods.
+ */
+export type AsyncPipeCondition<T> = boolean | ((subject: T) => Awaitable<boolean>)
+
+/**
+ * An asynchronous version of the Pipe helper.
+ */
+export class AsyncPipe<T> {
+    /**
+     * Get an AsyncPipe with the given value in it.
+     * @param subject
+     */
+    static wrap<subjectType>(subject: subjectType): AsyncPipe<subjectType> {
+        return new AsyncPipe<subjectType>(() => subject)
+    }
+
+    constructor(
+        /** The current value resolver of the pipe. */
+        private subject: AsyncPipeResolver<T>,
+    ) {}
+
+    /**
+     * Apply a transformative operator to the pipe.
+     * @param op
+     */
+    tap<T2>(op: AsyncPipeOperator<T, T2>): AsyncPipe<T2> {
+        return new AsyncPipe<T2>(async () => op(await this.subject()))
+    }
+
+    /**
+     * Apply an operator to the pipe, but return the reference
+     * to the current pipe. The operator is resolved when the
+     * overall pipe is resolved.
+     * @param op
+     */
+    peek<T2>(op: AsyncPipeOperator<T, T2>): AsyncPipe<T> {
+        return new AsyncPipe<T>(async () => {
+            const subject = await this.subject()
+            await op(subject)
+            return subject
+        })
+    }
+
+    /**
+     * Apply an operator to the pipe, if the check condition passes.
+     * @param check
+     * @param op
+     */
+    when(check: AsyncPipeCondition<T>, op: ReflexiveAsyncPipeOperator<T>): AsyncPipe<T> {
+        return new AsyncPipe<T>(async () => {
+            let subject
+
+            if ( typeof check === 'function' ) {
+                check = await check(subject = await this.subject())
+            }
+
+            subject = subject ?? await this.subject()
+            if ( check ) {
+                return ((await op(subject)) ?? subject) as T
+            }
+
+            return subject as T
+        })
+    }
+
+    /**
+     * Apply an operator to the pipe, if the check condition fails.
+     * @param check
+     * @param op
+     */
+    unless(check: AsyncPipeCondition<T>, op: ReflexiveAsyncPipeOperator<T>): AsyncPipe<T> {
+        if ( typeof check === 'function' ) {
+            return this.when(async (subject: T) => !(await check(subject)), op)
+        }
+
+        return this.when(!check, op)
+    }
+
+    /**
+     * Alias of `unless()`.
+     * @param check
+     * @param op
+     */
+    whenNot(check: AsyncPipeCondition<T>, op: ReflexiveAsyncPipeOperator<T>): AsyncPipe<T> {
+        return this.unless(check, op)
+    }
+
+    /**
+     * Get the transformed value from the pipe.
+     */
+    async resolve(): Promise<T> {
+        return this.subject()
+    }
+
+    /**
+     * Resolve the value and return it in a sync `Pipe` instance.
+     */
+    async sync(): Promise<Pipe<T>> {
+        return Pipe.wrap<T>(await this.subject())
+    }
+
+    /** Get the transformed value from the pipe. Allows awaiting the pipe directly. */
+    then(): Promise<T> {
+        return this.resolve()
+    }
 }

src/util/support/data.ts

@@ -1,6 +1,28 @@
 import * as nodeUUID from 'uuid'
 import {ErrorWithContext} from '../error/ErrorWithContext'
 import {JSONState} from './Rehydratable'
+import {KeyValue} from './types'
+
+/**
+ * Create an array of key-value pairs for the keys in a uniform object.
+ * @param obj
+ */
+export function objectToKeyValue<T>(obj: {[key: string]: T}): KeyValue<T>[] {
+    const values: KeyValue<T>[] = []
+
+    for ( const key in obj ) {
+        if ( !Object.prototype.hasOwnProperty.call(obj, key) ) {
+            continue
+        }
+
+        values.push({
+            key,
+            value: obj[key],
+        })
+    }
+
+    return values
+}
 
 /**
  * Make a deep copy of an object.

src/util/support/string.ts

@@ -47,3 +47,17 @@ export function padCenter(string: string, length: number, padWith = ' '): string
 
     return string
 }
+
+/**
+ * Convert a string to PascalCase.
+ * @param input
+ */
+export function stringToPascal(input: string): string {
+    return input.split(/[\s_]+/i)
+        .map(part => {
+            return part[0].toUpperCase() + part.substr(1)
+        })
+        .join('')
+        .split(/\W+/i)
+        .join('')
+}

src/util/support/types.ts

@@ -3,3 +3,9 @@ export type Awaitable<T> = T | Promise<T>
 
 /** Type alias for something that may be undefined. */
 export type Maybe<T> = T | undefined
+
+/** Type alias for a callback that accepts a typed argument. */
+export type ParameterizedCallback<T> = ((arg: T) => any)
+
+/** A key-value form of a given type. */
+export type KeyValue<T> = {key: string, value: T}