Backwards compatibility: Use DataClasses as companion

moor/example/example.g.dart

@@ -7,7 +7,7 @@ part of 'example.dart';
 // **************************************************************************
 
 // ignore_for_file: unnecessary_brace_in_string_interps
-class Category extends DataClass {
+class Category extends DataClass with DelegatingCompanionMixin<Category> {
   final int id;
   final String description;
   Category({this.id, this.description});
@@ -38,6 +38,16 @@ class Category extends DataClass {
     };
   }
 
+  @override
+  UpdateCompanion<Category> createCompanion(bool nullToAbsent) {
+    return CategoriesCompanion(
+      id: id == null && nullToAbsent ? const Value.absent() : Value.use(id),
+      description: description == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(description),
+    );
+  }
+
   Category copyWith({int id, String description}) => Category(
         id: id ?? this.id,
         description: description ?? this.description,
@@ -63,11 +73,11 @@ class CategoriesCompanion implements UpdateCompanion<Category> {
   final Value<int> id;
   final Value<String> description;
   const CategoriesCompanion({
-    this.id = Value.absent(),
+    this.id = const Value.absent(),
-    this.description = Value.absent(),
+    this.description = const Value.absent(),
   });
   @override
-  bool isValuePresent(int index) {
+  bool isValuePresent(int index, bool _) {
     switch (index) {
       case 0:
         return id.present;
@@ -151,7 +161,7 @@ class $CategoriesTable extends Categories
   }
 }
 
-class Recipe extends DataClass {
+class Recipe extends DataClass with DelegatingCompanionMixin<Recipe> {
   final int id;
   final String title;
   final String instructions;
@@ -192,6 +202,22 @@ class Recipe extends DataClass {
     };
   }
 
+  @override
+  UpdateCompanion<Recipe> createCompanion(bool nullToAbsent) {
+    return RecipesCompanion(
+      id: id == null && nullToAbsent ? const Value.absent() : Value.use(id),
+      title: title == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(title),
+      instructions: instructions == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(instructions),
+      category: category == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(category),
+    );
+  }
+
   Recipe copyWith({int id, String title, String instructions, int category}) =>
       Recipe(
         id: id ?? this.id,
@@ -231,13 +257,13 @@ class RecipesCompanion implements UpdateCompanion<Recipe> {
   final Value<String> instructions;
   final Value<int> category;
   const RecipesCompanion({
-    this.id = Value.absent(),
+    this.id = const Value.absent(),
-    this.title = Value.absent(),
+    this.title = const Value.absent(),
-    this.instructions = Value.absent(),
+    this.instructions = const Value.absent(),
-    this.category = Value.absent(),
+    this.category = const Value.absent(),
   });
   @override
-  bool isValuePresent(int index) {
+  bool isValuePresent(int index, bool _) {
     switch (index) {
       case 0:
         return id.present;
@@ -356,7 +382,7 @@ class $RecipesTable extends Recipes with TableInfo<$RecipesTable, Recipe> {
   }
 }
 
-class Ingredient extends DataClass {
+class Ingredient extends DataClass with DelegatingCompanionMixin<Ingredient> {
   final int id;
   final String name;
   final int caloriesPer100g;
@@ -391,6 +417,18 @@ class Ingredient extends DataClass {
     };
   }
 
+  @override
+  UpdateCompanion<Ingredient> createCompanion(bool nullToAbsent) {
+    return IngredientsCompanion(
+      id: id == null && nullToAbsent ? const Value.absent() : Value.use(id),
+      name:
+          name == null && nullToAbsent ? const Value.absent() : Value.use(name),
+      caloriesPer100g: caloriesPer100g == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(caloriesPer100g),
+    );
+  }
+
   Ingredient copyWith({int id, String name, int caloriesPer100g}) => Ingredient(
         id: id ?? this.id,
         name: name ?? this.name,
@@ -423,12 +461,12 @@ class IngredientsCompanion implements UpdateCompanion<Ingredient> {
   final Value<String> name;
   final Value<int> caloriesPer100g;
   const IngredientsCompanion({
-    this.id = Value.absent(),
+    this.id = const Value.absent(),
-    this.name = Value.absent(),
+    this.name = const Value.absent(),
-    this.caloriesPer100g = Value.absent(),
+    this.caloriesPer100g = const Value.absent(),
   });
   @override
-  bool isValuePresent(int index) {
+  bool isValuePresent(int index, bool _) {
     switch (index) {
       case 0:
         return id.present;
@@ -532,7 +570,8 @@ class $IngredientsTable extends Ingredients
   }
 }
 
-class IngredientInRecipe extends DataClass {
+class IngredientInRecipe extends DataClass
+    with DelegatingCompanionMixin<IngredientInRecipe> {
   final int recipe;
   final int ingredient;
   final int amountInGrams;
@@ -568,6 +607,21 @@ class IngredientInRecipe extends DataClass {
     };
   }
 
+  @override
+  UpdateCompanion<IngredientInRecipe> createCompanion(bool nullToAbsent) {
+    return IngredientInRecipesCompanion(
+      recipe: recipe == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(recipe),
+      ingredient: ingredient == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(ingredient),
+      amountInGrams: amountInGrams == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(amountInGrams),
+    );
+  }
+
   IngredientInRecipe copyWith(
           {int recipe, int ingredient, int amountInGrams}) =>
       IngredientInRecipe(
@@ -604,12 +658,12 @@ class IngredientInRecipesCompanion
   final Value<int> ingredient;
   final Value<int> amountInGrams;
   const IngredientInRecipesCompanion({
-    this.recipe = Value.absent(),
+    this.recipe = const Value.absent(),
-    this.ingredient = Value.absent(),
+    this.ingredient = const Value.absent(),
-    this.amountInGrams = Value.absent(),
+    this.amountInGrams = const Value.absent(),
   });
   @override
-  bool isValuePresent(int index) {
+  bool isValuePresent(int index, bool _) {
     switch (index) {
       case 0:
         return recipe.present;

moor/lib/src/runtime/data_class.dart

@@ -5,7 +5,7 @@ import 'package:moor/moor.dart';
 
 /// A common supertype for all data classes generated by moor. Data classes are
 /// immutable structures that represent a single row in a database table.
-abstract class DataClass implements UpdateCompanion {
+abstract class DataClass {
   const DataClass();
 
   /// Converts this object into a representation that can be encoded with
@@ -21,9 +21,6 @@ abstract class DataClass implements UpdateCompanion {
     return json.encode(toJson(serializer: serializer));
   }
 
-  @override
-  bool isValuePresent(int index) => true;
-
   /// Used internally be generated code
   @protected
   static dynamic parseJson(String jsonString) {
@@ -40,8 +37,37 @@ abstract class UpdateCompanion<D extends DataClass> {
   /// Used internally by moor.
   ///
   /// Returns true if the column at the position [index] has been explicitly
-  /// set to a value.
+  /// set to a value. The [assumeNullAsAbsent] parameter exists for backwards
-  bool isValuePresent(int index);
+  /// compatibility reasons, as [DataClass] implements [UpdateCompanion] but
+  /// doesn't have value fields.
+  bool isValuePresent(int index, bool assumeNullAsAbsent);
+}
+
+/// Used internally by moor for generated code.
+///
+/// Exists for backwards compatibility so that a [DataClass] can implement
+/// [UpdateCompanion].
+mixin DelegatingCompanionMixin<D extends DataClass>
+    implements UpdateCompanion<D> {
+  UpdateCompanion<D> _absent;
+  UpdateCompanion<D> _present;
+
+  @visibleForOverriding
+  UpdateCompanion<D> createCompanion(bool nullToAbsent);
+
+  UpdateCompanion<D> _resolveDelegate(bool nullToAbsent) {
+    if (nullToAbsent) {
+      return _absent ??= createCompanion(true);
+    } else {
+      return _present ??= createCompanion(false);
+    }
+  }
+
+  @override
+  bool isValuePresent(int index, bool assumeNullAsAbsent) {
+    final delegate = _resolveDelegate(assumeNullAsAbsent);
+    return delegate.isValuePresent(index, assumeNullAsAbsent);
+  }
 }
 
 /// A wrapper around arbitrary data [T] to indicate presence or absence

moor/test/data/tables/todos.g.dart

@@ -7,7 +7,7 @@ part of 'todos.dart';
 // **************************************************************************
 
 // ignore_for_file: unnecessary_brace_in_string_interps
-class TodoEntry extends DataClass {
+class TodoEntry extends DataClass with DelegatingCompanionMixin<TodoEntry> {
   final int id;
   final String title;
   final String content;
@@ -55,6 +55,25 @@ class TodoEntry extends DataClass {
     };
   }
 
+  @override
+  UpdateCompanion<TodoEntry> createCompanion(bool nullToAbsent) {
+    return TodosTableCompanion(
+      id: id == null && nullToAbsent ? const Value.absent() : Value.use(id),
+      title: title == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(title),
+      content: content == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(content),
+      targetDate: targetDate == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(targetDate),
+      category: category == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(category),
+    );
+  }
+
   TodoEntry copyWith(
           {int id,
           String title,
@@ -104,14 +123,14 @@ class TodosTableCompanion implements UpdateCompanion<TodoEntry> {
   final Value<DateTime> targetDate;
   final Value<int> category;
   const TodosTableCompanion({
-    this.id = Value.absent(),
+    this.id = const Value.absent(),
-    this.title = Value.absent(),
+    this.title = const Value.absent(),
-    this.content = Value.absent(),
+    this.content = const Value.absent(),
-    this.targetDate = Value.absent(),
+    this.targetDate = const Value.absent(),
-    this.category = Value.absent(),
+    this.category = const Value.absent(),
   });
   @override
-  bool isValuePresent(int index) {
+  bool isValuePresent(int index, bool _) {
     switch (index) {
       case 0:
         return id.present;
@@ -253,7 +272,7 @@ class $TodosTableTable extends TodosTable
   }
 }
 
-class Category extends DataClass {
+class Category extends DataClass with DelegatingCompanionMixin<Category> {
   final int id;
   final String description;
   Category({this.id, this.description});
@@ -284,6 +303,16 @@ class Category extends DataClass {
     };
   }
 
+  @override
+  UpdateCompanion<Category> createCompanion(bool nullToAbsent) {
+    return CategoriesCompanion(
+      id: id == null && nullToAbsent ? const Value.absent() : Value.use(id),
+      description: description == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(description),
+    );
+  }
+
   Category copyWith({int id, String description}) => Category(
         id: id ?? this.id,
         description: description ?? this.description,
@@ -309,11 +338,11 @@ class CategoriesCompanion implements UpdateCompanion<Category> {
   final Value<int> id;
   final Value<String> description;
   const CategoriesCompanion({
-    this.id = Value.absent(),
+    this.id = const Value.absent(),
-    this.description = Value.absent(),
+    this.description = const Value.absent(),
   });
   @override
-  bool isValuePresent(int index) {
+  bool isValuePresent(int index, bool _) {
     switch (index) {
       case 0:
         return id.present;
@@ -394,7 +423,7 @@ class $CategoriesTable extends Categories
   }
 }
 
-class User extends DataClass {
+class User extends DataClass with DelegatingCompanionMixin<User> {
   final int id;
   final String name;
   final bool isAwesome;
@@ -447,6 +476,24 @@ class User extends DataClass {
     };
   }
 
+  @override
+  UpdateCompanion<User> createCompanion(bool nullToAbsent) {
+    return UsersCompanion(
+      id: id == null && nullToAbsent ? const Value.absent() : Value.use(id),
+      name:
+          name == null && nullToAbsent ? const Value.absent() : Value.use(name),
+      isAwesome: isAwesome == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(isAwesome),
+      profilePicture: profilePicture == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(profilePicture),
+      creationTime: creationTime == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(creationTime),
+    );
+  }
+
   User copyWith(
           {int id,
           String name,
@@ -497,14 +544,14 @@ class UsersCompanion implements UpdateCompanion<User> {
   final Value<Uint8List> profilePicture;
   final Value<DateTime> creationTime;
   const UsersCompanion({
-    this.id = Value.absent(),
+    this.id = const Value.absent(),
-    this.name = Value.absent(),
+    this.name = const Value.absent(),
-    this.isAwesome = Value.absent(),
+    this.isAwesome = const Value.absent(),
-    this.profilePicture = Value.absent(),
+    this.profilePicture = const Value.absent(),
-    this.creationTime = Value.absent(),
+    this.creationTime = const Value.absent(),
   });
   @override
-  bool isValuePresent(int index) {
+  bool isValuePresent(int index, bool _) {
     switch (index) {
       case 0:
         return id.present;
@@ -642,7 +689,7 @@ class $UsersTable extends Users with TableInfo<$UsersTable, User> {
   }
 }
 
-class SharedTodo extends DataClass {
+class SharedTodo extends DataClass with DelegatingCompanionMixin<SharedTodo> {
   final int todo;
   final int user;
   SharedTodo({this.todo, this.user});
@@ -671,6 +718,16 @@ class SharedTodo extends DataClass {
     };
   }
 
+  @override
+  UpdateCompanion<SharedTodo> createCompanion(bool nullToAbsent) {
+    return SharedTodosCompanion(
+      todo:
+          todo == null && nullToAbsent ? const Value.absent() : Value.use(todo),
+      user:
+          user == null && nullToAbsent ? const Value.absent() : Value.use(user),
+    );
+  }
+
   SharedTodo copyWith({int todo, int user}) => SharedTodo(
         todo: todo ?? this.todo,
         user: user ?? this.user,
@@ -696,11 +753,11 @@ class SharedTodosCompanion implements UpdateCompanion<SharedTodo> {
   final Value<int> todo;
   final Value<int> user;
   const SharedTodosCompanion({
-    this.todo = Value.absent(),
+    this.todo = const Value.absent(),
-    this.user = Value.absent(),
+    this.user = const Value.absent(),
   });
   @override
-  bool isValuePresent(int index) {
+  bool isValuePresent(int index, bool _) {
     switch (index) {
       case 0:
         return todo.present;
@@ -785,7 +842,8 @@ class $SharedTodosTable extends SharedTodos
   }
 }
 
-class TableWithoutPKData extends DataClass {
+class TableWithoutPKData extends DataClass
+    with DelegatingCompanionMixin<TableWithoutPKData> {
   final int notReallyAnId;
   final double someFloat;
   TableWithoutPKData({this.notReallyAnId, this.someFloat});
@@ -818,6 +876,18 @@ class TableWithoutPKData extends DataClass {
     };
   }
 
+  @override
+  UpdateCompanion<TableWithoutPKData> createCompanion(bool nullToAbsent) {
+    return TableWithoutPKCompanion(
+      notReallyAnId: notReallyAnId == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(notReallyAnId),
+      someFloat: someFloat == null && nullToAbsent
+          ? const Value.absent()
+          : Value.use(someFloat),
+    );
+  }
+
   TableWithoutPKData copyWith({int notReallyAnId, double someFloat}) =>
       TableWithoutPKData(
         notReallyAnId: notReallyAnId ?? this.notReallyAnId,
@@ -847,11 +917,11 @@ class TableWithoutPKCompanion implements UpdateCompanion<TableWithoutPKData> {
   final Value<int> notReallyAnId;
   final Value<double> someFloat;
   const TableWithoutPKCompanion({
-    this.notReallyAnId = Value.absent(),
+    this.notReallyAnId = const Value.absent(),
-    this.someFloat = Value.absent(),
+    this.someFloat = const Value.absent(),
   });
   @override
-  bool isValuePresent(int index) {
+  bool isValuePresent(int index, bool _) {
     switch (index) {
       case 0:
         return notReallyAnId.present;

moor_generator/lib/src/writer/data_class_writer.dart

@@ -12,7 +12,8 @@ class DataClassWriter {
   DataClassWriter(this.table, this.options);
 
   void writeInto(StringBuffer buffer) {
-    buffer.write('class ${table.dartTypeName} extends DataClass {\n');
+    buffer.write('class ${table.dartTypeName} extends DataClass '
+        'with DelegatingCompanionMixin<${table.dartTypeName}> {\n');
 
     // write individual fields
     for (var column in table.columns) {
@@ -34,12 +35,12 @@ class DataClassWriter {
     // And a serializer and deserializer method
     _writeFromJson(buffer);
     _writeToJson(buffer);
+    _writeCompanionOverride(buffer);
 
     // And a convenience method to copy data from this class.
     _writeCopyWith(buffer);
 
     _writeToString(buffer);
-
     _writeHashCode(buffer);
 
     // override ==
@@ -217,6 +218,20 @@ class DataClassWriter {
     }
   }
 
+  void _writeCompanionOverride(StringBuffer buffer) {
+    // UpdateCompanion<D> createCompanion(bool nullToAbsent);
+    buffer.write('@override\nUpdateCompanion<${table.dartTypeName}> '
+        'createCompanion(bool nullToAbsent) {\n'
+        'return ${table.updateCompanionName}(');
+
+    for (var column in table.columns) {
+      final getter = column.dartGetterName;
+      buffer.write('$getter: $getter == null && nullToAbsent ? '
+          'const Value.absent() : Value.use($getter),');
+    }
+    buffer.write(');}\n');
+  }
+
   /// Recursively creates the implementation for hashCode of the data class,
   /// assuming it has at least one field. When it has one field, we just return
   /// the hash code of that field. Otherwise, we multiply it with 31 and add

moor_generator/lib/src/writer/update_companion_writer.dart

@@ -28,7 +28,7 @@ class UpdateCompanionWriter {
     buffer.write('const ${table.updateCompanionName}({');
 
     for (var column in table.columns) {
-      buffer.write('this.${column.dartGetterName} = Value.absent(),');
+      buffer.write('this.${column.dartGetterName} = const Value.absent(),');
     }
 
     buffer.write('});\n');
@@ -36,7 +36,7 @@ class UpdateCompanionWriter {
 
   void _writeIsPresentOverride(StringBuffer buffer) {
     buffer
-      ..write('@override\nbool isValuePresent(int index) {\n')
+      ..write('@override\nbool isValuePresent(int index, bool _) {\n')
       ..write('switch (index) {');
 
     for (var i = 0; i < table.columns.length; i++) {