Models unit; session model; generalize session classes/interfaces

orm/src/builder/type/ConnectionExecutable.ts

@@ -82,6 +82,10 @@ export default abstract class ConnectionExecutable<T> {
         return 0
     }
 
+    async exists(): Promise<boolean> {
+        return (await this.count()) > 0
+    }
+
     async execute_in_connection(connection: string | Connection): Promise<QueryResult> {
         const conn = typeof connection === 'string' ? make(Database).connection(connection) : connection