diff --git a/app/client/aclui/AccessRules.ts b/app/client/aclui/AccessRules.ts
index 8f9209c3..bf8a8a15 100644
--- a/app/client/aclui/AccessRules.ts
+++ b/app/client/aclui/AccessRules.ts
@@ -38,6 +38,7 @@ import {isHiddenCol} from 'app/common/gristTypes';
 import {isObject} from 'app/common/gutil';
 import * as roles from 'app/common/roles';
 import {SchemaTypes} from 'app/common/schema';
+import {MetaRowRecord} from 'app/common/TableData';
 import {ANONYMOUS_USER_EMAIL, EVERYONE_EMAIL, getRealAccess} from 'app/common/UserAPI';
 import {
   BaseObservable,
@@ -216,11 +217,11 @@ export class AccessRules extends Disposable {
     // ACL tables (they may have changed by other users). So our changes will win.
 
     const docData = this._gristDoc.docData;
-    const resourcesTable = docData.getTable('_grist_ACLResources')!;
-    const rulesTable = docData.getTable('_grist_ACLRules')!;
+    const resourcesTable = docData.getMetaTable('_grist_ACLResources');
+    const rulesTable = docData.getMetaTable('_grist_ACLRules');
 
     // Add/remove resources to have just the ones we need.
-    const newResources: RowRecord[] = flatten(
+    const newResources: MetaRowRecord<'_grist_ACLResources'>[] = flatten(
       [{tableId: '*', colIds: '*'}],
       this._specialRules.get()?.getResources() || [],
       ...this._tableRules.get().map(t => t.getResources()))
diff --git a/app/client/components/GristDoc.ts b/app/client/components/GristDoc.ts
index 6b5797b0..1c73d25c 100644
--- a/app/client/components/GristDoc.ts
+++ b/app/client/components/GristDoc.ts
@@ -675,7 +675,7 @@ export class GristDoc extends DisposableWithEvents {
   }
 
   public hasGranularAccessRules(): boolean {
-    const rulesTable = this.docData.getTable('_grist_ACLRules')!;
+    const rulesTable = this.docData.getMetaTable('_grist_ACLRules');
     // To check if there are rules, ignore the default no-op rule created for an older incarnation
     // of ACLs. It exists in older documents, and is still created for new ones. We detect it by
     // the use of the deprecated 'permissions' field, and not the new 'permissionsText' field.
diff --git a/app/client/components/duplicatePage.ts b/app/client/components/duplicatePage.ts
index 64db9561..6395b70f 100644
--- a/app/client/components/duplicatePage.ts
+++ b/app/client/components/duplicatePage.ts
@@ -90,7 +90,7 @@ async function updateViewSections(gristDoc: GristDoc, destViewSections: ViewSect
   const records: RowRecord[] = [];
   for (const srcViewSection of srcViewSections) {
     const viewSectionLayoutSpec = patchLayoutSpec(srcViewSection.layoutSpecObj.peek(), fieldsMap);
-    const record = gristDoc.docData.getTable('_grist_Views_section')!.getRecord(srcViewSection.getRowId())!;
+    const record = gristDoc.docData.getMetaTable('_grist_Views_section').getRecord(srcViewSection.getRowId())!;
     records.push({
       ...record,
       layoutSpec: JSON.stringify(viewSectionLayoutSpec),
@@ -126,7 +126,7 @@ async function updateViewFields(gristDoc: GristDoc, destViewSections: ViewSectio
     const srcViewFields: ViewFieldRec[] = srcViewSection!.viewFields.peek().peek();
     const parentId = destViewSection!.getRowId();
     for (const field of srcViewFields) {
-      const record = docData.getTable('_grist_Views_section_field')!.getRecord(field.getRowId())!;
+      const record = docData.getMetaTable('_grist_Views_section_field').getRecord(field.getRowId())!;
       fieldsToAdd.push({...record, parentId});
     }
   }
diff --git a/app/client/models/DocData.ts b/app/client/models/DocData.ts
index 68d24280..e7ca092c 100644
--- a/app/client/models/DocData.ts
+++ b/app/client/models/DocData.ts
@@ -5,10 +5,11 @@
  */
 
 import {DocComm} from 'app/client/components/DocComm';
-import {TableData} from 'app/client/models/TableData';
+import {MetaTableData, TableData} from 'app/client/models/TableData';
 import {ApplyUAOptions, ApplyUAResult} from 'app/common/ActiveDocAPI';
 import {CellValue, TableDataAction, UserAction} from 'app/common/DocActions';
 import {DocData as BaseDocData} from 'app/common/DocData';
+import {SchemaTypes} from 'app/common/schema';
 import {ColTypeMap} from 'app/common/TableData';
 import * as bluebird from 'bluebird';
 import {Emitter} from 'grainjs';
@@ -51,6 +52,11 @@ export class DocData extends BaseDocData {
     return super.getTable(tableId) as TableData;
   }
 
+  // Version of inherited getMetaTable() which returns the enhanced TableData type.
+  public getMetaTable<TableId extends keyof SchemaTypes>(tableId: TableId): MetaTableData<TableId> {
+    return super.getMetaTable(tableId) as any;
+  }
+
   /**
    * Finds up to n most likely target columns for the given values in the document.
    */
diff --git a/app/client/models/TableData.ts b/app/client/models/TableData.ts
index d0fd0ea2..62e58d46 100644
--- a/app/client/models/TableData.ts
+++ b/app/client/models/TableData.ts
@@ -1,19 +1,20 @@
 /**
  * TableData maintains a single table's data.
  */
-import { ColumnACIndexes } from 'app/client/models/ColumnACIndexes';
-import { ColumnCache } from 'app/client/models/ColumnCache';
-import { DocData } from 'app/client/models/DocData';
-import { DocAction, ReplaceTableData, TableDataAction, UserAction } from 'app/common/DocActions';
-import { isRaisedException } from 'app/common/gristTypes';
-import { countIf } from 'app/common/gutil';
-import { TableData as BaseTableData, ColTypeMap } from 'app/common/TableData';
-import { Emitter } from 'grainjs';
+import {ColumnACIndexes} from 'app/client/models/ColumnACIndexes';
+import {ColumnCache} from 'app/client/models/ColumnCache';
+import {DocData} from 'app/client/models/DocData';
+import {DocAction, ReplaceTableData, TableDataAction, UserAction} from 'app/common/DocActions';
+import {isRaisedException} from 'app/common/gristTypes';
+import {countIf} from 'app/common/gutil';
+import {SchemaTypes} from 'app/common/schema';
+import {ColTypeMap, MetaTableData as MetaTableDataBase, TableData as TableDataBase} from 'app/common/TableData';
+import {Emitter} from 'grainjs';
 
 /**
  * TableData class to maintain a single table's data.
  */
-export class TableData extends BaseTableData {
+export class TableData extends TableDataBase {
   public readonly tableActionEmitter = new Emitter();
   public readonly dataLoadedEmitter = new Emitter();
 
@@ -108,3 +109,5 @@ export class TableData extends BaseTableData {
     return applied;
   }
 }
+
+export type MetaTableData<TableId extends keyof SchemaTypes> = MetaTableDataBase<TableId> & TableData;
diff --git a/app/client/widgets/AttachmentsEditor.ts b/app/client/widgets/AttachmentsEditor.ts
index 42873ef4..fefb6631 100644
--- a/app/client/widgets/AttachmentsEditor.ts
+++ b/app/client/widgets/AttachmentsEditor.ts
@@ -7,7 +7,7 @@ import {dom, LiveIndex, makeLiveIndex, styled} from 'grainjs';
 import {DocComm} from 'app/client/components/DocComm';
 import {selectFiles, uploadFiles} from 'app/client/lib/uploads';
 import {DocData} from 'app/client/models/DocData';
-import {TableData} from 'app/client/models/TableData';
+import {MetaTableData} from 'app/client/models/TableData';
 import {basicButton, basicButtonLink, cssButtonGroup} from 'app/client/ui2018/buttons';
 import {colors, testId, vars} from 'app/client/ui2018/cssVars';
 import {editableLabel} from 'app/client/ui2018/editableLabel';
@@ -47,7 +47,7 @@ interface Attachment {
  * download, add or remove attachments in the edited cell.
  */
 export class AttachmentsEditor extends NewBaseEditor {
-  private _attachmentsTable: TableData;
+  private _attachmentsTable: MetaTableData<'_grist_Attachments'>;
   private _docComm: DocComm;
 
   private _rowIds: MutableObsArray<number>;
@@ -64,15 +64,15 @@ export class AttachmentsEditor extends NewBaseEditor {
     // editValue is abused slightly to indicate a 1-based index of the attachment.
     const initRowIndex: number|undefined = (options.editValue && parseInt(options.editValue, 0) - 1) || 0;
 
-    this._attachmentsTable = docData.getTable('_grist_Attachments')!;
+    this._attachmentsTable = docData.getMetaTable('_grist_Attachments');
     this._docComm = docData.docComm;
 
     this._rowIds = obsArray(Array.isArray(cellValue) ? cellValue.slice(1) as number[] : []);
     this._attachments = computedArray(this._rowIds, (val: number): Attachment => {
-      const fileIdent: string = this._attachmentsTable.getValue(val, 'fileIdent') as string;
+      const fileIdent: string = this._attachmentsTable.getValue(val, 'fileIdent')!;
       const fileType = mimeTypes.lookup(fileIdent) || 'application/octet-stream';
       const filename: Observable<string> =
-        observable(this._attachmentsTable.getValue(val, 'fileName') as string);
+        observable(this._attachmentsTable.getValue(val, 'fileName')!);
       return {
         rowId: val,
         fileIdent,
@@ -187,7 +187,7 @@ export class AttachmentsEditor extends NewBaseEditor {
   private async _renameAttachment(att: Attachment, fileName: string): Promise<void> {
     await this._attachmentsTable.sendTableAction(['UpdateRecord', att.rowId, {fileName}]);
     // Update the observable, since it's not on its own observing changes.
-    att.filename.set(this._attachmentsTable.getValue(att.rowId, 'fileName') as string);
+    att.filename.set(this._attachmentsTable.getValue(att.rowId, 'fileName')!);
   }
 
   private _getUrl(fileIdent: string, filename: string, inline?: boolean): string {
diff --git a/app/client/widgets/AttachmentsWidget.ts b/app/client/widgets/AttachmentsWidget.ts
index 5feab2b8..476b671b 100644
--- a/app/client/widgets/AttachmentsWidget.ts
+++ b/app/client/widgets/AttachmentsWidget.ts
@@ -7,7 +7,7 @@ import {cssRow} from 'app/client/ui/RightPanel';
 import {colors, vars} from 'app/client/ui2018/cssVars';
 import {NewAbstractWidget} from 'app/client/widgets/NewAbstractWidget';
 import {encodeQueryParams} from 'app/common/gutil';
-import {TableData} from 'app/common/TableData';
+import {MetaTableData} from 'app/client/models/TableData';
 import {UploadResult} from 'app/common/uploads';
 import {extname} from 'path';
 
@@ -66,7 +66,7 @@ export interface SavingObservable<T> extends ko.Observable<T> {
  */
 export class AttachmentsWidget extends NewAbstractWidget {
 
-  private _attachmentsTable: TableData;
+  private _attachmentsTable: MetaTableData<'_grist_Attachments'>;
   private _height: SavingObservable<string>;
 
   constructor(field: any) {
@@ -74,7 +74,7 @@ export class AttachmentsWidget extends NewAbstractWidget {
 
     // TODO: the Attachments table currently treated as metadata, and loaded on open,
     // but should probably be loaded on demand as it contains user data, which may be large.
-    this._attachmentsTable = this._getDocData().getTable('_grist_Attachments')!;
+    this._attachmentsTable = this._getDocData().getMetaTable('_grist_Attachments');
 
     this._height = this.options.prop('height') as SavingObservable<string>;
 
@@ -122,12 +122,12 @@ export class AttachmentsWidget extends NewAbstractWidget {
   }
 
   protected _buildAttachment(value: number, allValues: Computed<number[]>): Element {
-    const filename: string = this._attachmentsTable.getValue(value, 'fileName') as string;
-    const fileIdent: string = this._attachmentsTable.getValue(value, 'fileIdent') as string;
-    const height: number = this._attachmentsTable.getValue(value, 'imageHeight') as number;
-    const width: number = this._attachmentsTable.getValue(value, 'imageWidth') as number;
-    const hasPreview: boolean = Boolean(height);
-    const ratio: number = hasPreview ? (width / height) : 1;
+    const filename = this._attachmentsTable.getValue(value, 'fileName')!;
+    const fileIdent = this._attachmentsTable.getValue(value, 'fileIdent')!;
+    const height = this._attachmentsTable.getValue(value, 'imageHeight')!;
+    const width = this._attachmentsTable.getValue(value, 'imageWidth')!;
+    const hasPreview = Boolean(height);
+    const ratio = hasPreview ? (width / height) : 1;
 
     return attachmentPreview({title: filename}, // Add a filename tooltip to the previews.
       dom.style('height', (use) => `${use(this._height)}px`),
@@ -146,7 +146,7 @@ export class AttachmentsWidget extends NewAbstractWidget {
 
   // Returns the attachment download url.
   private _getUrl(rowId: number): string {
-    const ident = this._attachmentsTable.getValue(rowId, 'fileIdent') as string;
+    const ident = this._attachmentsTable.getValue(rowId, 'fileIdent');
     if (!ident) {
       return '';
     } else {
@@ -154,7 +154,7 @@ export class AttachmentsWidget extends NewAbstractWidget {
       return docComm.docUrl('attachment') + '?' + encodeQueryParams({
         ...docComm.getUrlParams(),
         ident,
-        name: this._attachmentsTable.getValue(rowId, 'fileName') as string
+        name: this._attachmentsTable.getValue(rowId, 'fileName')
       });
     }
   }
diff --git a/app/common/ACLRuleCollection.ts b/app/common/ACLRuleCollection.ts
index 67e8502f..bb418146 100644
--- a/app/common/ACLRuleCollection.ts
+++ b/app/common/ACLRuleCollection.ts
@@ -1,9 +1,9 @@
-import { parsePermissions } from 'app/common/ACLPermissions';
-import { ILogger } from 'app/common/BaseAPI';
-import { CellValue, RowRecord } from 'app/common/DocActions';
-import { DocData } from 'app/common/DocData';
-import { AclMatchFunc, ParsedAclFormula, RulePart, RuleSet, UserAttributeRule } from 'app/common/GranularAccessClause';
-import { getSetMapValue } from 'app/common/gutil';
+import {parsePermissions} from 'app/common/ACLPermissions';
+import {ILogger} from 'app/common/BaseAPI';
+import {DocData} from 'app/common/DocData';
+import {AclMatchFunc, ParsedAclFormula, RulePart, RuleSet, UserAttributeRule} from 'app/common/GranularAccessClause';
+import {getSetMapValue} from 'app/common/gutil';
+import {MetaRowRecord} from 'app/common/TableData';
 import sortBy = require('lodash/sortBy');
 
 const defaultMatchFunc: AclMatchFunc = () => true;
@@ -231,8 +231,8 @@ export class ACLRuleCollection {
    * Check that all references to table and column IDs in ACL rules are valid.
    */
   public checkDocEntities(docData: DocData) {
-    const tablesTable = docData.getTable('_grist_Tables')!;
-    const columnsTable = docData.getTable('_grist_Tables_column')!;
+    const tablesTable = docData.getMetaTable('_grist_Tables');
+    const columnsTable = docData.getMetaTable('_grist_Tables_column');
 
     // Collect valid tableIds and check rules against those.
     const validTableIds = new Set(tablesTable.getColValues('tableId'));
@@ -242,9 +242,9 @@ export class ACLRuleCollection {
     }
 
     // Collect valid columns, grouped by tableRef (rowId of table record).
-    const validColumns = new Map<number, Set<CellValue>>();   // Map from tableRef to set of colIds.
-    const colTableRefs = columnsTable.getColValues('parentId')!;
-    for (const [i, colId] of columnsTable.getColValues('colId')!.entries()) {
+    const validColumns = new Map<number, Set<string>>();   // Map from tableRef to set of colIds.
+    const colTableRefs = columnsTable.getColValues('parentId');
+    for (const [i, colId] of columnsTable.getColValues('colId').entries()) {
       getSetMapValue(validColumns, colTableRefs[i], () => new Set()).add(colId);
     }
 
@@ -302,14 +302,14 @@ export interface ReadAclResults {
  * UserAttributeRules. This is used by both client-side code and server-side.
  */
 function readAclRules(docData: DocData, {log, compile}: ReadAclOptions): ReadAclResults {
-  const resourcesTable = docData.getTable('_grist_ACLResources')!;
-  const rulesTable = docData.getTable('_grist_ACLRules')!;
+  const resourcesTable = docData.getMetaTable('_grist_ACLResources');
+  const rulesTable = docData.getMetaTable('_grist_ACLRules');
 
   const ruleSets: RuleSet[] = [];
   const userAttributes: UserAttributeRule[] = [];
 
   // Group rules by resource first, ordering by rulePos. Each group will become a RuleSet.
-  const rulesByResource = new Map<number, RowRecord[]>();
+  const rulesByResource = new Map<number, Array<MetaRowRecord<'_grist_ACLRules'>>>();
   for (const ruleRecord of sortBy(rulesTable.getRecords(), 'rulePos')) {
     getSetMapValue(rulesByResource, ruleRecord.resource, () => []).push(ruleRecord);
   }
@@ -325,8 +325,8 @@ function readAclRules(docData: DocData, {log, compile}: ReadAclOptions): ReadAcl
       // intentionally ignore and skip.
       continue;
     }
-    const tableId = resourceRec.tableId as string;
-    const colIds = resourceRec.colIds === '*' ? '*' : (resourceRec.colIds as string).split(',');
+    const tableId = resourceRec.tableId;
+    const colIds = resourceRec.colIds === '*' ? '*' : resourceRec.colIds.split(',');
 
     const body: RulePart[] = [];
     for (const rule of rules) {
diff --git a/app/common/GranularAccessClause.ts b/app/common/GranularAccessClause.ts
index 511ab4fb..b3e21657 100644
--- a/app/common/GranularAccessClause.ts
+++ b/app/common/GranularAccessClause.ts
@@ -1,6 +1,7 @@
-import { PartialPermissionSet } from 'app/common/ACLPermissions';
-import { CellValue, RowRecord } from 'app/common/DocActions';
-import { Role } from './roles';
+import {PartialPermissionSet} from 'app/common/ACLPermissions';
+import {CellValue, RowRecord} from 'app/common/DocActions';
+import {MetaRowRecord} from 'app/common/TableData';
+import {Role} from './roles';
 
 export interface RuleSet {
   tableId: '*' | string;
@@ -11,7 +12,7 @@ export interface RuleSet {
 }
 
 export interface RulePart {
-  origRecord?: RowRecord;         // Original record used to create this RulePart.
+  origRecord?: MetaRowRecord<'_grist_ACLRules'>;  // Original record used to create this RulePart.
   aclFormula: string;
   permissions: PartialPermissionSet;
   permissionsText: string;        // The text version of PermissionSet, as stored.
diff --git a/app/common/TableData.ts b/app/common/TableData.ts
index d9dcd86c..360c6817 100644
--- a/app/common/TableData.ts
+++ b/app/common/TableData.ts
@@ -13,7 +13,6 @@ import fromPairs = require('lodash/fromPairs');
 
 export interface ColTypeMap { [colId: string]: string; }
 
-type RowFunc<T> = (rowId: number) => T;
 type UIRowFunc<T> = (rowId: UIRowId) => T;
 
 interface ColData {
@@ -325,7 +324,7 @@ export class TableData extends ActionDispatcher implements SkippableRows {
    * Returns the first rowId matching the given filters, or 0 if no match. If there are multiple
    * matches, it is unspecified which will be returned.
    */
-  public findMatchingRowId(properties: {[key: string]: CellValue}): number {
+  public findMatchingRowId(properties: {[key: string]: CellValue | undefined}): number {
     const props = Object.keys(properties).map(p => ({col: this._columns.get(p)!, value: properties[p]}));
     if (!props.every((p) => p.col)) {
       return 0;
@@ -480,7 +479,15 @@ export class TableData extends ActionDispatcher implements SkippableRows {
   }
 }
 
-export type MetaRowRecord<TableId extends keyof SchemaTypes> = SchemaTypes[TableId] & RowRecord;
+// A type safe record of a meta table with types as defined in schema.ts
+// '&' is used because declaring the id field and the index signature in one block gives a syntax error.
+// The second part is basically equivalent to SchemaTypes[TableId]
+// but TS sees that as incompatible with RowRecord and doesn't allow simple overrides in MetaTableData.
+export type MetaRowRecord<TableId extends keyof SchemaTypes> =
+  { id: number } &
+  { [ColId in keyof SchemaTypes[TableId]]: SchemaTypes[TableId][ColId] & CellValue };
+
+type MetaColId<TableId extends keyof SchemaTypes> = keyof MetaRowRecord<TableId> & string;
 
 /**
  * Behaves the same as TableData, but uses SchemaTypes for type safety of its columns.
@@ -490,6 +497,11 @@ export class MetaTableData<TableId extends keyof SchemaTypes> extends TableData
     super(tableId, tableData, colTypes);
   }
 
+  public getValue<ColId extends MetaColId<TableId>>(rowId: number, colId: ColId):
+    MetaRowRecord<TableId>[ColId] | undefined {
+    return super.getValue(rowId, colId) as any;
+  }
+
   public getRecords(): Array<MetaRowRecord<TableId>> {
     return super.getRecords() as any;
   }
@@ -498,14 +510,31 @@ export class MetaTableData<TableId extends keyof SchemaTypes> extends TableData
     return super.getRecord(rowId) as any;
   }
 
-  /**
-   * Same as getRowPropFunc, but I couldn't get a direct override to compile.
-   */
-  public getMetaRowPropFunc<ColId extends keyof SchemaTypes[TableId]>(
+  public filterRecords(properties: Partial<MetaRowRecord<TableId>>): Array<MetaRowRecord<TableId>> {
+    return super.filterRecords(properties) as any;
+  }
+
+  public findMatchingRowId(properties: Partial<MetaRowRecord<TableId>>): number {
+    return super.findMatchingRowId(properties);
+  }
+
+  public getRowPropFunc<ColId extends MetaColId<TableId>>(
     colId: ColId
-  ): RowFunc<SchemaTypes[TableId][ColId]|undefined> {
+  ): UIRowFunc<MetaRowRecord<TableId>[ColId]> {
     return super.getRowPropFunc(colId as any) as any;
   }
+
+  public getColValues<ColId extends MetaColId<TableId>>(
+    colId: ColId
+  ): ReadonlyArray<MetaRowRecord<TableId>[ColId]> {
+    return super.getColValues(colId) as any;
+  }
+
+  public findRow<ColId extends MetaColId<TableId>>(
+    colId: ColId, colValue: MetaRowRecord<TableId>[ColId]
+  ): number {
+    return super.findRow(colId, colValue);
+  }
 }
 
 function reassignArray<T>(targetArray: T[], sourceArray: T[]): void {
diff --git a/app/server/lib/ActiveDoc.ts b/app/server/lib/ActiveDoc.ts
index 7af3dae5..d6e3b1e5 100644
--- a/app/server/lib/ActiveDoc.ts
+++ b/app/server/lib/ActiveDoc.ts
@@ -4,20 +4,9 @@
  * change events.
  */
 
-import { ActionSummary } from "app/common/ActionSummary";
-import { DocTriggers } from "app/server/lib/Triggers";
-import * as assert from 'assert';
-import {Mutex} from 'async-mutex';
-import * as bluebird from 'bluebird';
-import {EventEmitter} from 'events';
-import {IMessage, MsgType} from 'grain-rpc';
-import * as imageSize from 'image-size';
-import * as moment from 'moment-timezone';
-import fetch from 'node-fetch';
-import * as tmp from 'tmp';
-
 import {getEnvContent, LocalActionBundle, SandboxActionBundle, UserActionBundle} from 'app/common/ActionBundle';
-import { ActionGroup, MinimalActionGroup} from 'app/common/ActionGroup';
+import {ActionGroup, MinimalActionGroup} from 'app/common/ActionGroup';
+import {ActionSummary} from "app/common/ActionSummary";
 import {
   ApplyUAOptions,
   ApplyUAResult,
@@ -36,7 +25,6 @@ import {mapGetOrSet, MapWithTTL} from 'app/common/AsyncCreate';
 import {
   CellValue,
   DocAction,
-  RowRecord,
   TableDataAction,
   TableRecordValue,
   toTableDataAction,
@@ -50,6 +38,7 @@ import {FormulaProperties, getFormulaProperties} from 'app/common/GranularAccess
 import {byteString, countIf, safeJsonParse} from 'app/common/gutil';
 import {InactivityTimer} from 'app/common/InactivityTimer';
 import {schema, SCHEMA_VERSION} from 'app/common/schema';
+import {MetaRowRecord} from 'app/common/TableData';
 import {FetchUrlOptions, UploadResult} from 'app/common/uploads';
 import {DocReplacementOptions, DocState, DocStateComparison} from 'app/common/UserAPI';
 import {ParseOptions} from 'app/plugin/FileParserAPI';
@@ -66,7 +55,17 @@ import * as log from 'app/server/lib/log';
 import {LogMethods} from "app/server/lib/LogMethods";
 import {shortDesc} from 'app/server/lib/shortDesc';
 import {TableMetadataLoader} from 'app/server/lib/TableMetadataLoader';
+import {DocTriggers} from "app/server/lib/Triggers";
 import {fetchURL, FileUploadInfo, globalUploadSet, UploadInfo} from 'app/server/lib/uploads';
+import * as assert from 'assert';
+import {Mutex} from 'async-mutex';
+import * as bluebird from 'bluebird';
+import {EventEmitter} from 'events';
+import {IMessage, MsgType} from 'grain-rpc';
+import * as imageSize from 'image-size';
+import * as moment from 'moment-timezone';
+import fetch from 'node-fetch';
+import * as tmp from 'tmp';
 
 import {ActionHistory} from './ActionHistory';
 import {ActionHistoryImpl} from './ActionHistoryImpl';
@@ -89,8 +88,8 @@ import {findOrAddAllEnvelope, Sharing} from './Sharing';
 import cloneDeep = require('lodash/cloneDeep');
 import flatten = require('lodash/flatten');
 import remove = require('lodash/remove');
-import zipObject = require('lodash/zipObject');
 import without = require('lodash/without');
+import zipObject = require('lodash/zipObject');
 
 bluebird.promisifyAll(tmp);
 
@@ -608,14 +607,14 @@ export class ActiveDoc extends EventEmitter {
    * Returns the record from _grist_Attachments table for the given attachment ID,
    * or throws an error if not found.
    */
-  public getAttachmentMetadata(attId: number|string): RowRecord {
+  public getAttachmentMetadata(attId: number|string): MetaRowRecord<'_grist_Attachments'> {
     // docData should always be available after loadDoc() or createDoc().
     if (!this.docData) {
       throw new Error("No doc data");
     }
     // Parse strings into numbers to make more convenient to call from route handlers.
     const attachmentId: number = (typeof attId === 'string') ? parseInt(attId, 10) : attId;
-    const attRecord = this.docData.getTable('_grist_Attachments')!.getRecord(attachmentId);
+    const attRecord = this.docData.getMetaTable('_grist_Attachments').getRecord(attachmentId);
     if (!attRecord) {
       throw new ApiError(`Attachment not found: ${attId}`, 404);
     }
@@ -1116,14 +1115,14 @@ export class ActiveDoc extends EventEmitter {
       throw new Error('Cannot list ACL resources');
     }
     const result: {[tableId: string]: string[]} = {};
-    const tables = this.docData.getTable('_grist_Tables')!;
-    for (const tableId of tables.getColValues('tableId')!) {
-      result[tableId as string] = ['id'];
+    const tables = this.docData.getMetaTable('_grist_Tables');
+    for (const tableId of tables.getColValues('tableId')) {
+      result[tableId] = ['id'];
     }
-    const columns = this.docData.getTable('_grist_Tables_column')!;
+    const columns = this.docData.getMetaTable('_grist_Tables_column');
     for (const col of columns.getRecords()) {
-      const tableId = tables.getValue(col.parentId as number, 'tableId');
-      result[tableId as string].push(col.colId as string);
+      const tableId = tables.getValue(col.parentId, 'tableId')!;
+      result[tableId].push(col.colId);
     }
     return result;
   }
diff --git a/app/server/lib/DocPluginManager.ts b/app/server/lib/DocPluginManager.ts
index 5557bc38..5a8edc7c 100644
--- a/app/server/lib/DocPluginManager.ts
+++ b/app/server/lib/DocPluginManager.ts
@@ -33,8 +33,8 @@ class GristDocAPIImpl implements GristDocAPI {
   public async getDocName() { return this._activeDoc.docName; }
 
   public async listTables(): Promise<string[]> {
-    const table = this._activeDoc.docData!.getTable('_grist_Tables')!;
-    return (table.getColValues('tableId') as string[])
+    const table = this._activeDoc.docData!.getMetaTable('_grist_Tables');
+    return table.getColValues('tableId')
       .filter(id => !id.startsWith("GristSummary_")).sort();
   }
 
diff --git a/app/server/lib/ExpandedQuery.ts b/app/server/lib/ExpandedQuery.ts
index 0bf668ef..dbd7ee4d 100644
--- a/app/server/lib/ExpandedQuery.ts
+++ b/app/server/lib/ExpandedQuery.ts
@@ -56,8 +56,8 @@ export function expandQuery(iquery: ServerQuery, docData: DocData, onDemandFormu
   // Iterate through all formulas, adding joins and selects as we go.
   if (onDemandFormulas) {
     // Look up the main table for the query.
-    const tables = docData.getTable('_grist_Tables')!;
-    const columns = docData.getTable('_grist_Tables_column')!;
+    const tables = docData.getMetaTable('_grist_Tables');
+    const columns = docData.getMetaTable('_grist_Tables_column');
     const tableRef = tables.findRow('tableId', query.tableId);
     if (!tableRef) { throw new ApiError('table not found', 404); }
 
@@ -81,7 +81,7 @@ export function expandQuery(iquery: ServerQuery, docData: DocData, onDemandFormu
       let error = "";
       if (formula.kind === 'foreignColumn') {
         const altTableId = references.get(formula.refColId);
-        const altTableRef = tables.findRow('tableId', altTableId);
+        const altTableRef = tables.findRow('tableId', altTableId!);
         if (altTableId && altTableRef) {
           const altColumn = columns.filterRecords({parentId: altTableRef, isFormula: false, colId: formula.colId});
           // TODO: deal with a formula column in the other table.
diff --git a/app/server/lib/Export.ts b/app/server/lib/Export.ts
index 55c92833..ee35bab7 100644
--- a/app/server/lib/Export.ts
+++ b/app/server/lib/Export.ts
@@ -1,6 +1,5 @@
 import {ApiError} from 'app/common/ApiError';
 import {buildColFilter} from 'app/common/ColumnFilterFunc';
-import {RowRecord} from 'app/common/DocActions';
 import {DocData} from 'app/common/DocData';
 import {DocumentSettings} from 'app/common/DocumentSettings';
 import * as gristTypes from 'app/common/gristTypes';
@@ -9,7 +8,7 @@ import {buildRowFilter} from 'app/common/RowFilterFunc';
 import {SchemaTypes} from 'app/common/schema';
 import {SortFunc} from 'app/common/SortFunc';
 import {Sort} from 'app/common/SortSpec';
-import {TableData} from 'app/common/TableData';
+import {MetaRowRecord, MetaTableData} from 'app/common/TableData';
 import {ActiveDoc} from 'app/server/lib/ActiveDoc';
 import {RequestWithLogin} from 'app/server/lib/Authorizer';
 import {docSessionFromRequest} from 'app/server/lib/DocSession';
@@ -103,12 +102,14 @@ function safe<T>(value: T, msg: string) {
 }
 
 // Helper to for getting table from docData.
-const safeTable = (docData: DocData, name: keyof SchemaTypes) => safe(docData.getTable(name),
-  `No table '${name}' in document with id ${docData}`);
+function safeTable<TableId extends keyof SchemaTypes>(docData: DocData, name: TableId) {
+  return safe(docData.getMetaTable(name), `No table '${name}' in document with id ${docData}`);
+}
 
 // Helper for getting record safe
-const safeRecord = (table: TableData, id: number) => safe(table.getRecord(id),
-  `No record ${id} in table ${table.tableId}`);
+function safeRecord<TableId extends keyof SchemaTypes>(table: MetaTableData<TableId>, id: number) {
+  return safe(table.getRecord(id), `No record ${id} in table ${table.tableId}`);
+}
 
 /**
  * Builds export for all raw tables that are in doc.
@@ -138,9 +139,9 @@ export async function exportTable(
   req: express.Request): Promise<ExportData> {
   const docData = safe(activeDoc.docData, "No docData in active document");
   const tables = safeTable(docData, '_grist_Tables');
-  const table = safeRecord(tables, tableId) as GristTables;
-  const tableColumns = (safeTable(docData, '_grist_Tables_column')
-    .getRecords() as GristTablesColumn[])
+  const table = safeRecord(tables, tableId);
+  const tableColumns = safeTable(docData, '_grist_Tables_column')
+    .getRecords()
     // remove manual sort column
     .filter(col => col.colId !== gristTypes.MANUALSORT);
   // Produce a column description matching what user will see / expect to export
@@ -183,11 +184,11 @@ export async function exportTable(
   if (table.primaryViewId) {
     const viewId = table.primaryViewId;
     const views = safeTable(docData, '_grist_Views');
-    const view = safeRecord(views, viewId) as GristView;
+    const view = safeRecord(views, viewId);
     tableName = view.name;
   }
 
-  const docInfo = safeRecord(safeTable(docData, '_grist_DocInfo'), 1) as DocInfo;
+  const docInfo = safeRecord(safeTable(docData, '_grist_DocInfo'), 1);
   const docSettings = gutil.safeJsonParse(docInfo.documentSettings, {});
   return {
     tableName,
@@ -211,15 +212,15 @@ export async function exportSection(
 
   const docData = safe(activeDoc.docData, "No docData in active document");
   const viewSections = safeTable(docData, '_grist_Views_section');
-  const viewSection = safeRecord(viewSections, viewSectionId) as GristViewsSection;
+  const viewSection = safeRecord(viewSections, viewSectionId);
   const tables = safeTable(docData, '_grist_Tables');
-  const table = safeRecord(tables, viewSection.tableRef) as GristTables;
+  const table = safeRecord(tables, viewSection.tableRef);
   const columns = safeTable(docData, '_grist_Tables_column')
-    .filterRecords({ parentId: table.id }) as GristTablesColumn[];
+    .filterRecords({parentId: table.id});
   const viewSectionFields = safeTable(docData, '_grist_Views_section_field');
-  const fields = viewSectionFields.filterRecords({ parentId: viewSection.id }) as GristViewsSectionField[];
+  const fields = viewSectionFields.filterRecords({parentId: viewSection.id});
   const savedFilters = safeTable(docData, '_grist_Filters')
-    .filterRecords({ viewSectionRef: viewSection.id }) as GristFilter[];
+    .filterRecords({viewSectionRef: viewSection.id});
 
   const tableColsById = _.indexBy(columns, 'id');
   const fieldsByColRef = _.indexBy(fields, 'colRef');
@@ -279,7 +280,7 @@ export async function exportSection(
   // filter rows numbers
   rowIds = rowIds.filter(rowFilter);
 
-  const docInfo = safeRecord(safeTable(docData, '_grist_DocInfo'), 1) as DocInfo;
+  const docInfo = safeRecord(safeTable(docData, '_grist_DocInfo'), 1);
   const docSettings = gutil.safeJsonParse(docInfo.documentSettings, {});
 
   return {
@@ -292,17 +293,8 @@ export async function exportSection(
   };
 }
 
-// Type helpers for types used in this export
-type RowModel<TName extends keyof SchemaTypes> = RowRecord & {
-  [ColId in keyof SchemaTypes[TName]]: SchemaTypes[TName][ColId];
-};
-type GristViewsSection = RowModel<'_grist_Views_section'>
-type GristTables = RowModel<'_grist_Tables'>
-type GristViewsSectionField = RowModel<'_grist_Views_section_field'>
-type GristTablesColumn = RowModel<'_grist_Tables_column'>
-type GristView = RowModel<'_grist_Views'>
-type GristFilter = RowModel<'_grist_Filters'>
-type DocInfo = RowModel<'_grist_DocInfo'>
+type GristViewsSectionField = MetaRowRecord<'_grist_Views_section_field'>
+type GristTablesColumn = MetaRowRecord<'_grist_Tables_column'>
 
 // Type for filters passed from the client
 export interface Filter { colRef: number, filter: string }
diff --git a/app/server/lib/ExportCSV.ts b/app/server/lib/ExportCSV.ts
index f13230c4..46972175 100644
--- a/app/server/lib/ExportCSV.ts
+++ b/app/server/lib/ExportCSV.ts
@@ -74,7 +74,7 @@ export async function makeCSVFromTable(
   }
 
   // Look up the table to make a CSV from.
-  const tables = activeDoc.docData.getTable('_grist_Tables')!;
+  const tables = activeDoc.docData.getMetaTable('_grist_Tables');
   const tableRef = tables.findRow('tableId', tableId);
 
   if (tableRef === 0) {
diff --git a/app/server/lib/GranularAccess.ts b/app/server/lib/GranularAccess.ts
index 4270bbfb..925b3f4f 100644
--- a/app/server/lib/GranularAccess.ts
+++ b/app/server/lib/GranularAccess.ts
@@ -265,10 +265,10 @@ export class GranularAccess implements GranularAccessForBundle {
       // Create a tmpDocData with just the tables we care about, then update docActions to it.
       const tmpDocData: DocData = new DocData(
         (tableId) => { throw new Error("Unexpected DocData fetch"); }, {
-          _grist_Tables: this._docData.getTable('_grist_Tables')!.getTableDataAction(),
-          _grist_Tables_column: this._docData.getTable('_grist_Tables_column')!.getTableDataAction(),
-          _grist_ACLResources: this._docData.getTable('_grist_ACLResources')!.getTableDataAction(),
-          _grist_ACLRules: this._docData.getTable('_grist_ACLRules')!.getTableDataAction(),
+          _grist_Tables: this._docData.getMetaTable('_grist_Tables').getTableDataAction(),
+          _grist_Tables_column: this._docData.getMetaTable('_grist_Tables_column').getTableDataAction(),
+          _grist_ACLResources: this._docData.getMetaTable('_grist_ACLResources').getTableDataAction(),
+          _grist_ACLRules: this._docData.getMetaTable('_grist_ACLRules').getTableDataAction(),
         });
       for (const da of docActions) {
         tmpDocData.receiveAction(da);
diff --git a/app/server/lib/OnDemandActions.ts b/app/server/lib/OnDemandActions.ts
index 8dcd5831..78d2bd95 100644
--- a/app/server/lib/OnDemandActions.ts
+++ b/app/server/lib/OnDemandActions.ts
@@ -22,8 +22,8 @@ export interface OnDemandStorage {
  */
 export class OnDemandActions {
 
-  private _tablesMeta: TableData = this._docData.getTable('_grist_Tables')!;
-  private _columnsMeta: TableData = this._docData.getTable('_grist_Tables_column')!;
+  private _tablesMeta: TableData = this._docData.getMetaTable('_grist_Tables');
+  private _columnsMeta: TableData = this._docData.getMetaTable('_grist_Tables_column');
 
   constructor(private _storage: OnDemandStorage, private _docData: DocData) {}
 
diff --git a/app/server/lib/Triggers.ts b/app/server/lib/Triggers.ts
index a7fd05ed..5dd354ba 100644
--- a/app/server/lib/Triggers.ts
+++ b/app/server/lib/Triggers.ts
@@ -138,8 +138,8 @@ export class DocTriggers {
     }  // Happens on doc creation while processing InitNewDoc action.
 
     const triggersTable = docData.getMetaTable("_grist_Triggers");
-    const getTableId = docData.getMetaTable("_grist_Tables").getMetaRowPropFunc("tableId");
-    this._getColId = docData.getMetaTable("_grist_Tables_column").getMetaRowPropFunc("colId");
+    const getTableId = docData.getMetaTable("_grist_Tables").getRowPropFunc("tableId");
+    this._getColId = docData.getMetaTable("_grist_Tables_column").getRowPropFunc("colId");
 
     const triggersByTableRef = _.groupBy(triggersTable.getRecords(), "tableRef");
     const triggersByTableId: Array<[string, Trigger[]]> = [];