Relational models
DataStore has the capability to handle relationships between Models, such as has one, has many, belongs to. In GraphQL this is done with the @hasOne, @hasMany and @index directives as defined in the GraphQL Transformer documentation.
Updated schema
For the examples below with DataStore, let's add a new model to the schema.graphql file located by default at amplify/backend/{api_name}/:
enum PostStatus { ACTIVE INACTIVE}
type Post @model { id: ID! title: String! rating: Int! status: PostStatus! # New field with @hasMany comments: [Comment] @hasMany(indexName: "byPost", fields: ["id"])}
# New modeltype Comment @model { id: ID! postID: ID! @index(name: "byPost") post: Post! @belongsTo(fields: ["postID"]) content: String!}After that, regenerate the models with the following console command:
amplify codegen modelsSaving relations
In order to save connected models, you will create an instance of the model you wish to connect and pass its ID to DataStore.save:
import 'package:amplify_flutter/amplify_flutter.dart';
import 'models/ModelProvider.dart';
Future<void> savePostAndComment() async { final post = Post( title: 'My Post with comments', rating: 10, status: PostStatus.ACTIVE, ); final comment = Comment( post: post, // associate the comment to the post content: 'Loving Amplify DataStore!', );
try { // Make sure to safe the parent Post first await Amplify.DataStore.save(post); await Amplify.DataStore.save(comment); } on DataStoreException catch (e) { safePrint('Something went wrong querying posts: ${e.message}'); }}Querying relations
In order to retrieve all models that are related to a parent model, you can use query predicates with the query API:
import 'package:amplify_flutter/amplify_flutter.dart';
import 'models/ModelProvider.dart';
Future<void> fetchAllCommentsForPostId(String postId) async { try { final comments = await Amplify.DataStore.query( Comment.classType, where: Comment.POST.eq(postId), ); safePrint('Comments: $comments'); } on DataStoreException catch (e) { safePrint('Something went wrong querying posts: ${e.message}'); }}Deleting relations
When you delete a parent object in a one-to-many relationship, the children will also be removed from the DataStore and mutations for this deletion will be sent over the network. For example, the following operation would remove the Post with id 123 as well as any related comments:
import 'package:amplify_flutter/amplify_flutter.dart';
import 'models/ModelProvider.dart';
Future<void> deletePostWithID123AndItsComments(String id) async { try { final post = (await Amplify.DataStore.query( Post.classType, where: Post.ID.eq(id), )) .first;
// DataStore also deletes all associated comments with this operation await Amplify.DataStore.delete(post); } on DataStoreException catch (e) { safePrint('Something went wrong deleting models: ${e.message}'); }}However, in a many-to-many relationship the children are not removed and you must explicitly delete them.
Many-to-many
For many-to-many relationships, you can use the @manyToMany directive and specify a relationName. Under the hood, Amplify creates a join table and a one-to-many relationship from both models.
enum PostStatus { ACTIVE INACTIVE}
type Post @model { id: ID! title: String! rating: Int status: PostStatus editors: [User] @manyToMany(relationName: "PostEditor")}
type User @model { id: ID! username: String! posts: [Post] @manyToMany(relationName: "PostEditor")}Add the model above to the schema.graphql file located by default at amplify/backend/{api_name}/ and regenerate the models again with the following command:
amplify codegen modelsOnce it is regenerated, save your posts with many-to-many mode like the following:
import 'package:amplify_flutter/amplify_flutter.dart';
import 'models/ModelProvider.dart';
Future<void> savePostAndEditor() async { final post = Post( title: 'My First Post', rating: 10, status: PostStatus.INACTIVE, ); final editor = User(username: 'Nadia'); final postEditor = PostEditor(post: post, user: editor);
try { // first you save the post await Amplify.DataStore.save(post);
// secondly, you save the editor/user await Amplify.DataStore.save(editor);
// then you save the model that links a post with an editor await Amplify.DataStore.save(postEditor); } on DataStoreException catch (e) { safePrint('Something went wrong saving model: ${e.message}'); }}