Allow async mappings from SQL to row classes

When existing, custom row classes are used, drift now supports using a
(potentially asynchronous) static method to load them instead of just
a named constructor like before.
Tables are also changed to support the `map` method being async for
cases where that is needed. The same applies to custom queries which
may have to be async now.

docs/build.yaml

@@ -100,4 +100,4 @@ targets:
 global_options:
   ":api_index":
     options:
-      packages: ['drift', 'drift_dev']
+      packages: ['drift', 'drift_dev', 'sqlite3']

docs/lib/snippets/custom_row_classes/default.dart

@@ -0,0 +1,18 @@
+import 'package:drift/drift.dart';
+
+// #docregion start
+@UseRowClass(User)
+class Users extends Table {
+  IntColumn get id => integer().autoIncrement()();
+  TextColumn get name => text()();
+  DateTimeColumn get birthday => dateTime()();
+}
+
+class User {
+  final int id;
+  final String name;
+  final DateTime birthday;
+
+  User({required this.id, required this.name, required this.birthday});
+}
+// #enddocregion start

docs/lib/snippets/custom_row_classes/named.dart

@@ -0,0 +1,21 @@
+import 'package:drift/drift.dart';
+
+// #docregion named
+@UseRowClass(User, constructor: 'fromDb')
+class Users extends Table {
+  // ...
+  // #enddocregion named
+  IntColumn get id => integer().autoIncrement()();
+  TextColumn get name => text()();
+  DateTimeColumn get birthday => dateTime()();
+  // #docregion named
+}
+
+class User {
+  final int id;
+  final String name;
+  final DateTime birthday;
+
+  User.fromDb({required this.id, required this.name, required this.birthday});
+}
+// #enddocregion named

docs/pages/docs/Advanced Features/custom_row_classes.md

@@ -6,6 +6,7 @@ data:
 template: layouts/docs/single
 ---
 
+
 For each table declared in Dart or in a drift file, `drift_dev` generates a row class (sometimes also referred to as _data class_)
 to hold a full row and a companion class for updates and inserts.
 This works well for most cases: Drift knows  what columns your table has, and it can generate a simple class for all of that.
@@ -19,22 +20,8 @@ Starting from moor version 4.3 (and in drift), it is possible to use your own cl
 
 To use a custom row class, simply annotate your table definition with `@UseRowClass`.
 
-```dart
+{% assign snippets = "package:drift_docs/snippets/custom_row_classes/default.dart.excerpt.json" | readString | json_decode %}
-@UseRowClass(User)
+{% include "blocks/snippet" snippets = snippets name = "start" %}
-class Users extends Table {
-  IntColumn get id => integer().autoIncrement()();
-  TextColumn get name => text()();
-  DateTimeColumn get birthday => dateTime()();
-}
-
-class User {
-  final int id;
-  final String name;
-  final DateTime birthDate;
-
-  User({required this.id, required this.name, required this.birthDate});
-}
-```
 
 A row class must adhere to the following requirements:
 
@@ -58,18 +45,25 @@ By default, drift will use the default, unnamed constructor to map a row to the
 If you want to use another constructor, set the `constructor` parameter on the
 `@UseRowClass` annotation:
 
+{% assign snippets = "package:drift_docs/snippets/custom_row_classes/named.dart.excerpt.json" | readString | json_decode %}
+{% include "blocks/snippet" snippets = snippets name = "named" %}
+
+### Static and aynchronous factories
+
+Starting with drift 2.0, the custom constructor set with the `constructor`
+parameter on the `@UseRowClass` annotation may also refer to a static method
+defined on the class to load.
+That method must either return the row class or a `Future` of that type.
+Unlike a named constructor or a factory, this can be useful in case the mapping
+from SQL to Dart needs to be asynchronous:
+
 ```dart
-@UseRowClass(User, constructor: 'fromDb')
-class Users extends Table {
-  // ...
-}
-
 class User {
-  final int id;
+  // ...
-  final String name;
-  final DateTime birthDate;
 
-  User.fromDb({required this.id, required this.name, required this.birthDate});
+  static Future<User> load(int id, String name, DateTime birthday) async {
+    // ...
+  }
 }
 ```
 

drift/CHANGELOG.md

@@ -17,6 +17,8 @@
 - __Breaking__: Remove the `includeJoinedTableColumns` parameter on `selectOnly()`.
   The method now behaves as if that parameter was turned off. To use columns from a
   joined table, add them with `addColumns`.
+- __Breaking__: Remove the `fromData` factory on generated data classes. Use the
+  `map` method on tables instead.
 - Add support for storing date times as (ISO-8601) strings. For details on how
   to use this, see [the documentation](https://drift.simonbinder.eu/docs/getting-started/advanced_dart_tables/#supported-column-types).
 - Consistently handle transaction errors like a failing `BEGIN` or `COMMIT`
@@ -25,6 +27,9 @@
   to delete statatements.
 - Support nested transactions.
 - Support custom collations in the query builder API.
+- [Custom row classes](https://drift.simonbinder.eu/docs/advanced-features/custom_row_classes/)
+  can now be constructed with static methods too.
+  These static factories can also be asynchronous.
 
 ## 1.7.1
 

drift/lib/src/dsl/columns.dart

@@ -324,6 +324,9 @@ extension BuildGeneralColumn<T extends Object> on _BaseColumnBuilder<T> {
   /// ```
   /// The generated row class will then use a `MyFancyClass` instead of a
   /// `String`, which would usually be used for [Table.text] columns.
+  ///
+  /// The type [T] of the type converter may only be nullable if this column
+  /// was declared [nullable] too. Otherwise, `drift_dev` will emit an error.
   ColumnBuilder<T> map<Dart>(TypeConverter<Dart, T?> converter) =>
       _isGenerated();
 

drift/lib/src/runtime/query_builder/query_builder.dart

@@ -10,6 +10,8 @@ import 'package:drift/src/runtime/executor/stream_queries.dart';
 import 'package:drift/src/utils/single_transformer.dart';
 import 'package:meta/meta.dart';
 
+import '../../utils/async.dart';
+
 // New files should not be part of this mega library, which we're trying to
 // split up.
 import 'expressions/case_when.dart';

drift/lib/src/runtime/query_builder/schema/entities.dart

@@ -93,7 +93,7 @@ abstract class ResultSetImplementation<Tbl, Row> extends DatabaseSchemaEntity {
   List<GeneratedColumn> get $columns;
 
   /// Maps the given row returned by the database into the fitting data class.
-  Row map(Map<String, dynamic> data, {String? tablePrefix});
+  FutureOr<Row> map(Map<String, dynamic> data, {String? tablePrefix});
 
   /// Creates an alias of this table or view that will write the name [alias]
   /// when used in a query.
@@ -125,7 +125,7 @@ class _AliasResultSet<Tbl, Row> extends ResultSetImplementation<Tbl, Row> {
   String get entityName => _inner.entityName;
 
   @override
-  Row map(Map<String, dynamic> data, {String? tablePrefix}) {
+  FutureOr<Row> map(Map<String, dynamic> data, {String? tablePrefix}) {
     return _inner.map(data, tablePrefix: tablePrefix);
   }
 

drift/lib/src/runtime/query_builder/schema/table_info.dart

@@ -75,7 +75,8 @@ mixin TableInfo<TableDsl extends Table, D> on Table
   /// The [database] instance is used so that the raw values from the companion
   /// can properly be interpreted as the high-level Dart values exposed by the
   /// data class.
-  D mapFromCompanion(Insertable<D> companion, DatabaseConnectionUser database) {
+  Future<D> mapFromCompanion(
+      Insertable<D> companion, DatabaseConnectionUser database) async {
     final asColumnMap = companion.toColumns(false);
 
     if (asColumnMap.values.any((e) => e is! Variable)) {
@@ -122,20 +123,20 @@ mixin VirtualTableInfo<TableDsl extends Table, D> on TableInfo<TableDsl, D> {
 /// Most of these are accessed internally by drift or by generated code.
 extension TableInfoUtils<TableDsl, D> on ResultSetImplementation<TableDsl, D> {
   /// Like [map], but from a [row] instead of the low-level map.
-  D mapFromRow(QueryRow row, {String? tablePrefix}) {
+  Future<D> mapFromRow(QueryRow row, {String? tablePrefix}) async {
     return map(row.data, tablePrefix: tablePrefix);
   }
 
   /// Like [mapFromRow], but returns null if a non-nullable column of this table
   /// is null in [row].
-  D? mapFromRowOrNull(QueryRow row, {String? tablePrefix}) {
+  Future<D?> mapFromRowOrNull(QueryRow row, {String? tablePrefix}) {
     final resolvedPrefix = tablePrefix == null ? '' : '$tablePrefix.';
 
     final notInRow = $columns
         .where((c) => !c.$nullable)
         .any((e) => row.data['$resolvedPrefix${e.$name}'] == null);
 
-    if (notInRow) return null;
+    if (notInRow) return Future.value(null);
 
     return mapFromRow(row, tablePrefix: tablePrefix);
   }
@@ -153,7 +154,7 @@ extension TableInfoUtils<TableDsl, D> on ResultSetImplementation<TableDsl, D> {
   ///
   /// Drift would generate code to call this method with `'c1': 'foo'` and
   /// `'c2': 'bar'` in [alias].
-  D mapFromRowWithAlias(QueryRow row, Map<String, String> alias) {
+  Future<D> mapFromRowWithAlias(QueryRow row, Map<String, String> alias) async {
     return map({
       for (final entry in row.data.entries) alias[entry.key]!: entry.value,
     });

drift/lib/src/runtime/query_builder/statements/delete.dart

@@ -78,7 +78,7 @@ class DeleteStatement<T extends Table, D> extends Query<T, D>
             {TableUpdate.onTable(_sourceTable, kind: UpdateKind.delete)});
       }
 
-      return [for (final rawRow in rows) table.map(rawRow)];
+      return rows.mapAsyncAndAwait(table.map);
     });
   }
 }

drift/lib/src/runtime/query_builder/statements/select/select.dart

@@ -60,7 +60,7 @@ class SimpleSelectStatement<T extends HasResultSet, D> extends Query<T, D>
       key: StreamKey(query.sql, query.boundVariables),
     );
 
-    return database.createStream(fetcher).map(_mapResponse);
+    return database.createStream(fetcher).asyncMap(_mapResponse);
   }
 
   Future<List<Map<String, Object?>>> _getRaw(GenerationContext ctx) {
@@ -69,8 +69,8 @@ class SimpleSelectStatement<T extends HasResultSet, D> extends Query<T, D>
     });
   }
 
-  List<D> _mapResponse(List<Map<String, Object?>> rows) {
+  Future<List<D>> _mapResponse(List<Map<String, Object?>> rows) {
-    return rows.map(table.map).toList();
+    return rows.mapAsyncAndAwait(table.map);
   }
 
   /// Creates a select statement that operates on more than one table by

drift/lib/src/runtime/query_builder/statements/select/select_with_join.dart

@@ -207,7 +207,7 @@ class JoinedSelectStatement<FirstT extends HasResultSet, FirstD>
 
     return database
         .createStream(fetcher)
-        .map((rows) => _mapResponse(ctx, rows));
+        .asyncMap((rows) => _mapResponse(ctx, rows));
   }
 
   @override
@@ -234,9 +234,9 @@ class JoinedSelectStatement<FirstT extends HasResultSet, FirstD>
     });
   }
 
-  List<TypedResult> _mapResponse(
+  Future<List<TypedResult>> _mapResponse(
       GenerationContext ctx, List<Map<String, Object?>> rows) {
-    return rows.map((row) {
+    return Future.wait(rows.map((row) async {
       final readTables = <ResultSetImplementation, dynamic>{};
       final readColumns = <Expression, dynamic>{};
 
@@ -244,7 +244,8 @@ class JoinedSelectStatement<FirstT extends HasResultSet, FirstD>
         final prefix = '${table.aliasedName}.';
         // if all columns of this table are null, skip the table
         if (table.$columns.any((c) => row[prefix + c.$name] != null)) {
-          readTables[table] = table.map(row, tablePrefix: table.aliasedName);
+          readTables[table] =
+              await table.map(row, tablePrefix: table.aliasedName);
         }
       }
 
@@ -256,7 +257,7 @@ class JoinedSelectStatement<FirstT extends HasResultSet, FirstD>
       }
 
       return TypedResult(readTables, QueryRow(row, database), readColumns);
-    }).toList();
+    }));
   }
 
   @alwaysThrows

drift/lib/src/runtime/query_builder/statements/update.dart

@@ -95,7 +95,7 @@ class UpdateStatement<T extends Table, D> extends Query<T, D>
           {TableUpdate.onTable(_sourceTable, kind: UpdateKind.update)});
     }
 
-    return [for (final rawRow in rows) table.map(rawRow)];
+    return rows.mapAsyncAndAwait(table.map);
   }
 
   /// Replaces the old version of [entity] that is stored in the database with

drift/lib/src/utils/async.dart

@@ -0,0 +1,12 @@
+@internal
+import 'dart:async';
+
+import 'package:meta/meta.dart';
+
+/// Drift-internal utilities to map potentially async operations.
+extension MapAndAwait<T> on Iterable<T> {
+  /// A variant of [Future.wait] that also works for [FutureOr].
+  Future<List<R>> mapAsyncAndAwait<R>(FutureOr<R> Function(T) mapper) {
+    return Future.wait(map((e) => Future.sync(() => mapper(e))));
+  }
+}

drift/test/database/tables_test.dart

@@ -34,20 +34,20 @@ void main() {
     expect(aliasA.hashCode == db.alias(db.users, 'a').hashCode, isTrue);
   });
 
-  test('can convert a companion to a row class', () {
+  test('can convert a companion to a row class', () async {
     const companion = SharedTodosCompanion(
       todo: Value(3),
       user: Value(4),
     );
 
-    final user = db.sharedTodos.mapFromCompanion(companion, db);
+    final user = await db.sharedTodos.mapFromCompanion(companion, db);
     expect(
       user,
       const SharedTodo(todo: 3, user: 4),
     );
   });
 
-  test('can map from row without table prefix', () {
+  test('can map from row without table prefix', () async {
     final rowData = {
       'id': 1,
       'title': 'some title',
@@ -55,7 +55,7 @@ void main() {
       'target_date': null,
       'category': null,
     };
-    final todo = db.todosTable.mapFromRowOrNull(QueryRow(rowData, db));
+    final todo = await db.todosTable.mapFromRowOrNull(QueryRow(rowData, db));
     expect(
       todo,
       const TodoEntry(

drift/test/generated/custom_tables.g.dart

@@ -1596,7 +1596,7 @@ abstract class _$CustomTablesDb extends GeneratedDatabase {
         ],
         readsFrom: {
           config,
-        }).map((QueryRow row) => config.mapFromRowWithAlias(row, const {
+        }).asyncMap((QueryRow row) => config.mapFromRowWithAlias(row, const {
           'ck': 'config_key',
           'cf': 'config_value',
           'cs1': 'sync_state',
@@ -1621,7 +1621,7 @@ abstract class _$CustomTablesDb extends GeneratedDatabase {
         readsFrom: {
           config,
           ...generatedclause.watchedTables,
-        }).map(config.mapFromRow);
+        }).asyncMap(config.mapFromRow);
   }
 
   Selectable<Config> readDynamic(
@@ -1638,7 +1638,7 @@ abstract class _$CustomTablesDb extends GeneratedDatabase {
         readsFrom: {
           config,
           ...generatedpredicate.watchedTables,
-        }).map(config.mapFromRow);
+        }).asyncMap(config.mapFromRow);
   }
 
   Selectable<String> typeConverterVar(SyncType? var1, List<SyncType?> var2,
@@ -1710,12 +1710,12 @@ abstract class _$CustomTablesDb extends GeneratedDatabase {
           withDefaults,
           withConstraints,
           ...generatedpredicate.watchedTables,
-        }).map((QueryRow row) {
+        }).asyncMap((QueryRow row) async {
       return MultipleResult(
         row: row,
         a: row.readNullable<String>('a'),
         b: row.readNullable<int>('b'),
-        c: withConstraints.mapFromRowOrNull(row, tablePrefix: 'nested_0'),
+        c: await withConstraints.mapFromRowOrNull(row, tablePrefix: 'nested_0'),
       );
     });
   }
@@ -1728,7 +1728,7 @@ abstract class _$CustomTablesDb extends GeneratedDatabase {
         ],
         readsFrom: {
           email,
-        }).map(email.mapFromRow);
+        }).asyncMap(email.mapFromRow);
   }
 
   Selectable<ReadRowIdResult> readRowId(
@@ -1762,7 +1762,7 @@ abstract class _$CustomTablesDb extends GeneratedDatabase {
   Selectable<MyViewData> readView() {
     return customSelect('SELECT * FROM my_view', variables: [], readsFrom: {
       config,
-    }).map(myView.mapFromRow);
+    }).asyncMap(myView.mapFromRow);
   }
 
   Selectable<int> cfeTest() {
@@ -1786,9 +1786,10 @@ abstract class _$CustomTablesDb extends GeneratedDatabase {
         $writeInsertable(this.config, value, startIndex: $arrayStartIndex);
     $arrayStartIndex += generatedvalue.amountOfVariables;
     return customWriteReturning(
-        'INSERT INTO config ${generatedvalue.sql} RETURNING *',
+            'INSERT INTO config ${generatedvalue.sql} RETURNING *',
-        variables: [...generatedvalue.introducedVariables],
+            variables: [...generatedvalue.introducedVariables],
-        updates: {config}).then((rows) => rows.map(config.mapFromRow).toList());
+            updates: {config})
+        .then((rows) => Future.wait(rows.map(config.mapFromRow)));
   }
 
   Selectable<NestedResult> nested(String? var1) {
@@ -1803,7 +1804,7 @@ abstract class _$CustomTablesDb extends GeneratedDatabase {
         }).asyncMap((QueryRow row) async {
       return NestedResult(
         row: row,
-        defaults: withDefaults.mapFromRow(row, tablePrefix: 'nested_0'),
+        defaults: await withDefaults.mapFromRow(row, tablePrefix: 'nested_0'),
         nestedQuery0: await customSelect(
             'SELECT * FROM with_constraints AS c WHERE c.b = ?1',
             variables: [
@@ -1812,7 +1813,7 @@ abstract class _$CustomTablesDb extends GeneratedDatabase {
             readsFrom: {
               withConstraints,
               withDefaults,
-            }).map(withConstraints.mapFromRow).get(),
+            }).asyncMap(withConstraints.mapFromRow).get(),
       );
     });
   }

drift/test/generated/todos.g.dart

@@ -1642,7 +1642,7 @@ abstract class _$TodoDb extends GeneratedDatabase {
         ],
         readsFrom: {
           todosTable,
-        }).map(todosTable.mapFromRow);
+        }).asyncMap(todosTable.mapFromRow);
   }
 
   Selectable<TodoEntry> search({required int id}) {
@@ -1653,7 +1653,7 @@ abstract class _$TodoDb extends GeneratedDatabase {
         ],
         readsFrom: {
           todosTable,
-        }).map(todosTable.mapFromRow);
+        }).asyncMap(todosTable.mapFromRow);
   }
 
   Selectable<MyCustomObject> findCustom() {
@@ -1750,6 +1750,6 @@ mixin _$SomeDaoMixin on DatabaseAccessor<TodoDb> {
           todosTable,
           sharedTodos,
           users,
-        }).map(todosTable.mapFromRow);
+        }).asyncMap(todosTable.mapFromRow);
   }
 }

drift/test/test_utils/test_utils.dart

@@ -53,5 +53,7 @@ class CustomTable extends Table with TableInfo<CustomTable, Null> {
   }
 
   @override
-  Null map(Map<String, dynamic> data, {String? tablePrefix}) => null;
+  Future<Null> map(Map<String, dynamic> data, {String? tablePrefix}) async {
+    return null;
+  }
 }

drift_dev/lib/src/analyzer/dart_types.dart

@@ -28,10 +28,13 @@ ExistingRowClass? validateExistingClass(
     Step step) {
   final errors = step.errors;
   final desiredClass = dartClass.classElement;
-  ConstructorElement? ctor;
+  final library = desiredClass.library;
+
+  ExecutableElement? ctor;
+  final InterfaceType instantiation;
 
   if (dartClass.instantiation != null) {
-    final instantiation = desiredClass.instantiate(
+    instantiation = desiredClass.instantiate(
       typeArguments: dartClass.instantiation!,
       nullabilitySuffix: NullabilitySuffix.none,
     );
@@ -41,6 +44,36 @@ ExistingRowClass? validateExistingClass(
     ctor = instantiation.lookUpConstructor(constructor, desiredClass.library);
   } else {
     ctor = desiredClass.getNamedConstructor(constructor);
+    instantiation = library.typeSystem.instantiateInterfaceToBounds(
+        element: desiredClass, nullabilitySuffix: NullabilitySuffix.none);
+  }
+
+  if (ctor == null) {
+    final fallback = desiredClass.getMethod(constructor);
+
+    if (fallback != null) {
+      if (!fallback.isStatic) {
+        errors.report(ErrorInDartCode(
+          affectedElement: fallback,
+          message: 'To use this method as a factory for the custom row class, '
+              'it needs to be static.',
+        ));
+      }
+
+      // The static factory must return a subtype of `FutureOr<ThatRowClass>`
+      final expectedReturnType =
+          library.typeProvider.futureOrType(instantiation);
+      if (!library.typeSystem
+          .isAssignableTo(fallback.returnType, expectedReturnType)) {
+        errors.report(ErrorInDartCode(
+          affectedElement: fallback,
+          message: 'To be used as a factory for the custom row class, this '
+              'method needs to return an instance of it.',
+        ));
+      }
+
+      ctor = fallback;
+    }
   }
 
   if (ctor == null) {

drift_dev/lib/src/model/base_entity.dart

@@ -69,15 +69,31 @@ class ExistingRowClass {
 
   /// The Dart types that should be used to instantiate the [targetClass].
   final List<DartType> typeInstantiation;
-  final ConstructorElement constructor;
+
+  /// The method to use when instantiating the row class.
+  ///
+  /// This may either be a constructor or a static method on the row class.
+  final ExecutableElement constructor;
+
   final Map<DriftColumn, ParameterElement> mapping;
 
   /// Generate toCompanion for data class
   final bool generateInsertable;
 
   ExistingRowClass(
-      this.targetClass, this.constructor, this.mapping, this.generateInsertable,
+    this.targetClass,
-      {this.typeInstantiation = const []});
+    this.constructor,
+    this.mapping,
+    this.generateInsertable, {
+    this.typeInstantiation = const [],
+  });
+
+  /// Whether the [constructor] returns a future and thus needs to be awaited
+  /// to create an instance of the custom row class.
+  bool get isAsyncFactory {
+    final typeSystem = targetClass.library.typeSystem;
+    return typeSystem.flatten(constructor.returnType) != constructor.returnType;
+  }
 
   String dartType([GenerationOptions options = const GenerationOptions()]) {
     if (typeInstantiation.isEmpty) {

drift_dev/lib/src/model/sql_query.dart

@@ -118,6 +118,17 @@ abstract class SqlQuery {
     placeholders = elements.whereType<FoundDartPlaceholder>().toList();
   }
 
+  bool get needsAsyncMapping {
+    final result = resultSet;
+    if (result != null) {
+      // Mapping to tables is asynchronous
+      if (result.matchingTable != null) return true;
+      if (result.nestedResults.any((e) => e is NestedResultTable)) return true;
+    }
+
+    return false;
+  }
+
   String get resultClassName {
     final resultSet = this.resultSet;
     if (resultSet == null) {
@@ -194,6 +205,9 @@ class SqlSelectQuery extends SqlQuery {
   bool get hasNestedQuery =>
       resultSet.nestedResults.any((e) => e is NestedResultQuery);
 
+  @override
+  bool get needsAsyncMapping => hasNestedQuery || super.needsAsyncMapping;
+
   SqlSelectQuery(
     String name,
     this.fromContext,

drift_dev/lib/src/writer/queries/query_writer.dart

@@ -115,7 +115,7 @@ class QueryWriter {
       }
     } else {
       _buffer.write('(QueryRow row) ');
-      if (query is SqlSelectQuery && query.hasNestedQuery) {
+      if (query.needsAsyncMapping) {
         _buffer.write('async ');
       }
       _buffer.write('{ return ${query.resultClassName}(');
@@ -140,7 +140,7 @@ class QueryWriter {
           final mappingMethod =
               nested.isNullable ? 'mapFromRowOrNull' : 'mapFromRow';
 
-          _buffer.write('$fieldName: $tableGetter.$mappingMethod(row, '
+          _buffer.write('$fieldName: await $tableGetter.$mappingMethod(row, '
               'tablePrefix: ${asDartLiteral(prefix)}),');
         } else if (nested is NestedResultQuery) {
           final fieldName = nested.filedName();
@@ -173,8 +173,8 @@ class QueryWriter {
         // The type converter maps non-nullable types, but the column may be
         // nullable in SQL => just map null to null and only invoke the type
         // converter for non-null values.
-        code = 'NullAwareTypeConverter.wrapFromSql(${_converter(converter)}, '
+        code = 'NullAwareTypeConverter.wrapFromSql'
-            '$code)';
+            '(${_converter(converter)}, $code)';
       } else {
         // Just apply the type converter directly.
         code = '${_converter(converter)}.fromSql($code)';
@@ -206,7 +206,7 @@ class QueryWriter {
     _buffer.write(', ');
     _writeReadsFrom(select);
 
-    if (select.hasNestedQuery) {
+    if (select.needsAsyncMapping) {
       _buffer.write(').asyncMap(');
     } else {
       _buffer.write(').map(');
@@ -259,9 +259,18 @@ class QueryWriter {
     _writeExpandedDeclarations(update);
     _buffer.write('return customWriteReturning(${_queryCode(update)},');
     _writeCommonUpdateParameters(update);
-    _buffer.write(').then((rows) => rows.map(');
+
-    _writeMappingLambda(update);
+    _buffer.write(').then((rows) => ');
-    _buffer.write(').toList());\n}');
+    if (update.needsAsyncMapping) {
+      _buffer.write('Future.wait(rows.map(');
+      _writeMappingLambda(update);
+      _buffer.write('))');
+    } else {
+      _buffer.write('rows.map(');
+      _writeMappingLambda(update);
+      _buffer.write(')');
+    }
+    _buffer.write(');\n}');
   }
 
   void _writeUpdatingQuery(UpdatingQuery update) {

drift_dev/lib/src/writer/tables/table_writer.dart

@@ -136,9 +136,13 @@ abstract class TableOrViewWriter {
 
     final dataClassName = tableOrView.dartTypeCode();
 
+    final isAsync = tableOrView.existingRowClass?.isAsyncFactory == true;
+    final returnType = isAsync ? 'Future<$dataClassName>' : dataClassName;
+    final asyncModifier = isAsync ? 'async' : '';
+
     buffer
-      ..write('@override\n$dataClassName map(Map<String, dynamic> data, '
+      ..write('@override $returnType map(Map<String, dynamic> data, '
-          '{String? tablePrefix}) {\n')
+          '{String? tablePrefix}) $asyncModifier {\n')
       ..write('final effectivePrefix = '
           "tablePrefix != null ? '\$tablePrefix.' : '';");
 
@@ -174,6 +178,7 @@ abstract class TableOrViewWriter {
       final ctor = info.constructor;
       buffer
         ..write('return ')
+        ..write(isAsync ? 'await ' : '')
         ..write(classElement.name);
       if (ctor.name.isNotEmpty) {
         buffer

drift_dev/test/analyzer/dart/custom_row_classes_test.dart

@@ -148,10 +148,50 @@ class Cls extends HasBar {
 
   Cls(this.foo, int bar): super(bar);
 }
+''',
+      'a|lib/async_factory.dart': '''
+import 'package:drift/drift.dart';
+
+@UseRowClass(MyCustomClass, constructor: 'load')
+class Tbl extends Table {
+  TextColumn get foo => text()();
+  IntColumn get bar => integer()();
+}
+
+class MyCustomClass {
+  static Future<MyCustomClass> load(String foo, int bar) async {
+    throw 'stub';
+  }
+}
+''',
+      'a|lib/invalid_static_factory.dart': '''
+import 'package:drift/drift.dart';
+
+@UseRowClass(MyCustomClass, constructor: 'invalidReturn')
+class Tbl extends Table {
+  TextColumn get foo => text()();
+  IntColumn get bar => integer()();
+}
+
+@UseRowClass(MyCustomClass, constructor: 'notStatic')
+class Tbl2 extends Table {
+  TextColumn get foo => text()();
+  IntColumn get bar => integer()();
+}
+
+class MyCustomClass {
+  static String invalidReturn(String foo, int bar) async {
+    throw 'stub';
+  }
+
+  MyRowClass notStatic() {
+    throw 'stub';
+  }
+}
 ''',
       'a|lib/custom_parent_class_no_error.dart': '''
 import 'package:drift/drift.dart';
-  
+
 abstract class BaseModel extends DataClass {
   abstract final String id;
 }
@@ -164,7 +204,7 @@ class Companies extends Table {
 ''',
       'a|lib/custom_parent_class_typed_no_error.dart': '''
 import 'package:drift/drift.dart';
-  
+
 abstract class BaseModel<T> extends DataClass {
   abstract final String id;
 }
@@ -177,7 +217,7 @@ class Companies extends Table {
 ''',
       'a|lib/custom_parent_class_no_super.dart': '''
 import 'package:drift/drift.dart';
-  
+
 abstract class BaseModel {
   abstract final String id;
 }
@@ -190,7 +230,7 @@ class Companies extends Table {
 ''',
       'a|lib/custom_parent_class_wrong_super.dart': '''
 import 'package:drift/drift.dart';
-  
+
 class Test {
 }
 
@@ -206,7 +246,7 @@ class Companies extends Table {
 ''',
       'a|lib/custom_parent_class_typed_wrong_type_arg.dart': '''
 import 'package:drift/drift.dart';
-  
+
 abstract class BaseModel<T> extends DataClass {
   abstract final String id;
 }
@@ -219,7 +259,7 @@ class Companies extends Table {
 ''',
       'a|lib/custom_parent_class_two_type_argument.dart': '''
 import 'package:drift/drift.dart';
-  
+
 abstract class BaseModel<T, D> extends DataClass {
   abstract final String id;
 }
@@ -234,7 +274,7 @@ class Companies extends Table {
 import 'package:drift/drift.dart';
 
 typedef NotClass = void Function();
-  
+
 @DataClassName('Company', extending: NotClass)
 class Companies extends Table {
   TextColumn get id => text()();
@@ -306,6 +346,35 @@ class Companies extends Table {
             contains('but some are missing: bar'))),
       );
     });
+
+    test('for invalid static factories', () async {
+      final file = await state.analyze('package:a/invalid_static_factory.dart');
+
+      expect(
+          file.errors.errors,
+          allOf(
+            contains(
+              isA<ErrorInDartCode>()
+                  .having(
+                    (e) => e.message,
+                    'message',
+                    contains('it needs to be static'),
+                  )
+                  .having((e) => e.affectedElement?.name,
+                      'affectedElement.name', 'notStatic'),
+            ),
+            contains(
+              isA<ErrorInDartCode>()
+                  .having(
+                    (e) => e.message,
+                    'message',
+                    contains('needs to return an instance of it'),
+                  )
+                  .having((e) => e.affectedElement?.name,
+                      'affectedElement.name', 'invalidReturn'),
+            ),
+          ));
+    });
   });
 
   test('supports generic row classes', () async {
@@ -361,6 +430,17 @@ class Companies extends Table {
     expect(file.errors.errors, isEmpty);
   });
 
+  test('supports async factories for existing row classes', () async {
+    final file = await state.analyze('package:a/async_factory.dart');
+    expect(file.errors.errors, isEmpty);
+
+    final table = file.currentResult!.declaredTables.single;
+    expect(
+        table.existingRowClass,
+        isA<ExistingRowClass>()
+            .having((e) => e.isAsyncFactory, 'isAsyncFactory', isTrue));
+  });
+
   group('custom data class parent', () {
     test('check valid', () async {
       final file =

drift_dev/test/writer/data_class_writer_test.dart

@@ -9,10 +9,17 @@ import 'package:drift_dev/src/backends/build/drift_builder.dart';
 import 'package:pub_semver/pub_semver.dart';
 import 'package:test/test.dart';
 
-const _testInput = r'''
+void main() {
+  test(
+    'generates const constructor for data classes can companion classes',
+    () async {
+      await testBuilder(
+        DriftPartBuilder(const BuilderOptions({}), isForNewDriftPackage: true),
+        const {
+          'a|lib/main.dart': r'''
 import 'package:drift/drift.dart';
 
-part 'main.moor.dart';
+part 'main.drift.dart';
 
 class Users extends Table {
   IntColumn get id => integer().autoIncrement()();
@@ -23,18 +30,11 @@ class Users extends Table {
   tables: [Users],
 )
 class Database extends _$Database {}
-''';
+'''
-
+        },
-void main() {
-  test(
-    'generates const constructor for data classes can companion classes',
-    () async {
-      await testBuilder(
-        DriftPartBuilder(const BuilderOptions({})),
-        const {'a|lib/main.dart': _testInput},
         reader: await PackageAssetReader.currentIsolate(),
         outputs: const {
-          'a|lib/main.moor.dart': _GeneratesConstDataClasses(
+          'a|lib/main.drift.dart': _GeneratesConstDataClasses(
             {'User', 'UsersCompanion'},
           ),
         },
@@ -42,6 +42,56 @@ void main() {
     },
     tags: 'analyzer',
   );
+
+  test(
+    'generates async mapping code for existing row class with async factory',
+    () async {
+      await testBuilder(
+        DriftPartBuilder(const BuilderOptions({}), isForNewDriftPackage: true),
+        const {
+          'a|lib/main.dart': r'''
+import 'package:drift/drift.dart';
+
+part 'main.drift.dart';
+
+@UseRowClass(MyCustomClass, constructor: 'load')
+class Tbl extends Table {
+  TextColumn get foo => text()();
+  IntColumn get bar => integer()();
+}
+
+class MyCustomClass {
+  static Future<MyCustomClass> load(String foo, int bar) async {
+    throw 'stub';
+  }
+}
+
+@DriftDatabase(
+  tables: [Tbl],
+)
+class Database extends _$Database {}
+'''
+        },
+        reader: await PackageAssetReader.currentIsolate(),
+        outputs: {
+          'a|lib/main.drift.dart': decodedMatches(contains(r'''
+  @override
+  Future<MyCustomClass> map(Map<String, dynamic> data,
+      {String? tablePrefix}) async {
+    final effectivePrefix = tablePrefix != null ? '$tablePrefix.' : '';
+    return await MyCustomClass.load(
+      attachedDatabase.options.types
+          .read(DriftSqlType.string, data['${effectivePrefix}foo'])!,
+      attachedDatabase.options.types
+          .read(DriftSqlType.int, data['${effectivePrefix}bar'])!,
+    );
+  }
+''')),
+        },
+      );
+    },
+    tags: 'analyzer',
+  );
 }
 
 class _GeneratesConstDataClasses extends Matcher {

examples/app/lib/database/database.g.dart

@@ -651,10 +651,10 @@ abstract class _$AppDatabase extends GeneratedDatabase {
           textEntries,
           todoEntries,
           categories,
-        }).map((QueryRow row) {
+        }).asyncMap((QueryRow row) async {
       return SearchResult(
-        todos: todoEntries.mapFromRow(row, tablePrefix: 'nested_0'),
+        todos: await todoEntries.mapFromRow(row, tablePrefix: 'nested_0'),
-        cat: categories.mapFromRowOrNull(row, tablePrefix: 'nested_1'),
+        cat: await categories.mapFromRowOrNull(row, tablePrefix: 'nested_1'),
       );
     });
   }

examples/flutter_web_worker_example/lib/src/database/database.g.dart

@@ -182,7 +182,7 @@ abstract class _$MyDatabase extends GeneratedDatabase {
   Selectable<Entrie> allEntries() {
     return customSelect('SELECT * FROM entries', variables: [], readsFrom: {
       entries,
-    }).map(entries.mapFromRow);
+    }).asyncMap(entries.mapFromRow);
   }
 
   Future<int> addEntry(String var1) {

examples/web_worker_example/lib/database.g.dart

@@ -183,7 +183,7 @@ abstract class _$MyDatabase extends GeneratedDatabase {
   Selectable<Entrie> allEntries() {
     return customSelect('SELECT * FROM entries', variables: [], readsFrom: {
       entries,
-    }).map(entries.mapFromRow);
+    }).asyncMap(entries.mapFromRow);
   }
 
   Future<int> addEntry(String var1) {

extras/integration_tests/drift_testcases/lib/database/database.g.dart

@@ -296,7 +296,7 @@ class $UsersTable extends Users with TableInfo<$UsersTable, User> {
   @override
   Set<GeneratedColumn> get $primaryKey => {id};
   @override
-  User map(Map<String, dynamic> data, {String? tablePrefix}) {
+  Future<User> map(Map<String, dynamic> data, {String? tablePrefix}) async {
     final effectivePrefix = tablePrefix != null ? '$tablePrefix.' : '';
     return User(
       id: attachedDatabase.options.types
@@ -526,7 +526,8 @@ class $FriendshipsTable extends Friendships
   @override
   Set<GeneratedColumn> get $primaryKey => {firstUser, secondUser};
   @override
-  Friendship map(Map<String, dynamic> data, {String? tablePrefix}) {
+  Future<Friendship> map(Map<String, dynamic> data,
+      {String? tablePrefix}) async {
     final effectivePrefix = tablePrefix != null ? '$tablePrefix.' : '';
     return Friendship(
       firstUser: attachedDatabase.options.types
@@ -558,7 +559,7 @@ abstract class _$Database extends GeneratedDatabase {
         readsFrom: {
           users,
           friendships,
-        }).map(users.mapFromRow);
+        }).asyncMap(users.mapFromRow);
   }
 
   Selectable<int> amountOfGoodFriends(int user) {
@@ -581,10 +582,10 @@ abstract class _$Database extends GeneratedDatabase {
         readsFrom: {
           friendships,
           users,
-        }).map((QueryRow row) {
+        }).asyncMap((QueryRow row) async {
       return FriendshipsOfResult(
         reallyGoodFriends: row.read<bool>('really_good_friends'),
-        user: users.mapFromRow(row, tablePrefix: 'nested_0'),
+        user: await users.mapFromRow(row, tablePrefix: 'nested_0'),
       );
     });
   }
@@ -618,7 +619,7 @@ abstract class _$Database extends GeneratedDatabase {
         ],
         readsFrom: {
           users,
-        }).map(users.mapFromRow);
+        }).asyncMap(users.mapFromRow);
   }
 
   Future<List<Friendship>> returning(int var1, int var2, bool var3) {
@@ -631,7 +632,7 @@ abstract class _$Database extends GeneratedDatabase {
         ],
         updates: {
           friendships
-        }).then((rows) => rows.map(friendships.mapFromRow).toList());
+        }).then((rows) => Future.wait(rows.map(friendships.mapFromRow)));
   }
 
   @override