Manipulating data

Amplify Flutter v0 is now in Maintenance Mode until July 19th, 2024. This means that we will continue to include updates to ensure compatibility with backend services and security. No new features will be introduced in v0.

Please use the latest version (v1) of Amplify Flutter to get started.

If you are currently using v0, follow these instructions to upgrade to v1.

Create and update

To write data to the DataStore, pass an instance of a model to Amplify.DataStore.save():

1Future<void> savePost() async {
2  final newPost = Post(
3    title: 'New Post being saved',
4    rating: 15,
5    status: PostStatus.INACTIVE,
6  );
7
8  await Amplify.DataStore.save(newPost);
9}

The save method creates a new record, or in the event that one already exists in the local store, it updates the record.

1Future<void> updatePost() async {
2  final postsWithId = await Amplify.DataStore.query(
3    Post.classType,
4    where: Post.ID.eq('123'),
5  );
6
7  final oldPost = postsWithId.first;
8  final newPost = oldPost.copyWith(id: oldPost.id, title: 'Updated Title');
9
10  await Amplify.DataStore.save(newPost);
11}

Avoid working with stale data!

Model instances which store values, such as those from the results of a DataStore.Query operation, can become stale and outdated when their properties are updated. This can be the result of a manual change or even a side effect of real time data being received by the application. In order to ensure you are performing mutations on the latest committed state to the system, either perform a query directly before the DataStore.save() operation or observe the model to keep the state updated at all times and perform mutations on that latest state referencing the model instance. The preceding example demonstrates one approach. The following example demonstrates the observeQuery approach.

1class MyApp extends StatefulWidget {
2  const MyApp({Key? key}) : super(key: key);
3
4  @override
5  State<MyApp> createState() => _MyAppState();
6}
7
8class _MyAppState extends State<MyApp> {
9  StreamSubscription<QuerySnapshot<Post>>? _stream;
10
11  // A reference to the retrieved post
12  late Post _post;
13
14  @override
15  void initState() {
16    super.initState();
17    _configure();
18  }
19  
20  // Initialize the Amplify libraries and call `observeQuery`
21  Future<void> _configure() async {
22    // ...
23  }
24  ...
25
26  void observeQuery() {
27    _stream = Amplify.DataStore.observeQuery(
28      Post.classType,
29      where: Post.ID.eq('123')
30    ).listen((QuerySnapshot<Post> snapshot) {
31      setState(() {
32        _post = snapshot.items.first;
33      });
34    });
35  }
36
37  Future<void> updatePost(String newTitle) async {
38      final updatedPost = _post.copyWith(title: newTitle);
39      await Amplify.DataStore.save(updatedPost);
40
41      // you do not need to explicitly set _post here; observeQuery will see
42      // the update and set the variable.
43  }
44
45  // Build function and UI elements
46  ...
47
48  @override
49  void dispose() {
50    _stream?.cancel();
51    super.dispose();
52  }
53}

Delete

To delete an item, simply pass in an instance.

Below, you query for an instance with an id of 123, and then delete it, if found:

1Future<void> deletePostsWithId() async {
2  final oldPosts = await Amplify.DataStore.query(
3    Post.classType,
4    where: Post.ID.eq('123'),
5  );
6  // Query can return more than one posts with a different predicate
7  // For this example, it is ensured that it will return one post
8  final oldPost = oldPosts.first;
9  try {
10    await Amplify.DataStore.delete(oldPost);
11    print('Deleted a post');
12  } on DataStoreException catch (e) {
13    print('Delete failed: $e');
14  }
15}

Query Data

Queries are performed against the local store. When cloud synchronization is enabled, the local store is updated in the background by the DataStore Sync Engine.

For more advanced filtering, such as matching arbitrary field values on an object, you can supply a query predicate.

1Future<void> readFromDatabase() async {
2  try {
3    final posts = await Amplify.DataStore.query(Post.classType);
4    print('Posts: $posts');
5  } on DataStoreException catch (e) {
6    print('Query failed: $e');
7  }
8}

Predicates

Predicates are filters that can be used to match items in the DataStore. When applied to a query(), they constrain the returned results. When applied to a save(), they act as a pre-requisite for updating the data. You can match against fields in your schema by using the following predicates:

Strings: eq | ne | le | lt | ge | gt | contains | beginsWith | between

Numbers: eq | ne | le | lt | ge | gt | between

Lists: contains

For example if you wanted a list of all Post Models that have a rating greater than 4:

1Future<void> fetchPostsMoreThanFourRating() async {
2  final posts = await Amplify.DataStore.query(
3    Post.classType,
4    where: Post.RATING.ge(4),
5  );
6  print('Posts: $posts');
7}

Multiple conditions can also be used, like the ones defined in GraphQL Transform condition statements. For example, fetch all posts that have a rating greater than 4 and are ACTIVE:

1Future<void> fetchPublishedWithRatingTwoPosts() async {
2  final posts = await Amplify.DataStore.query(
3    Post.classType,
4    where: Post.RATING.eq(2).and(Post.STATUS.eq(PostStatus.ACTIVE)),
5  );
6  print('Posts: $posts');
7}

Alternatively, the or logical operator can also be used:

1Future<void> fetchPublishedOrWithRatingTwoPosts() async {
2  final posts = await Amplify.DataStore.query(
3    Post.classType,
4    where: Post.RATING.eq(2).or(Post.STATUS.eq(PostStatus.ACTIVE)),
5  );
6  print('Posts: $posts');
7}

Sort

Query results can also be sorted by one or more fields.

For example, to sort all Post objects by rating in ascending order:

1Future<void> fetchPostsInAscendingOrder() async {
2  final posts = await Amplify.DataStore.query(
3    Post.classType,
4    sortBy: [Post.RATING.ascending()],
5  );
6  print('Posts: $posts');
7}

To get all Post objects sorted first by rating in ascending order, and then by title in descending order:

1Future<void> fetchPostsFirstInAscendingRatingOrderThenDescendingTitleOrder() async {
2  final posts = await Amplify.DataStore.query(
3    Post.classType,
4    sortBy: [
5      Post.RATING.ascending(),
6      Post.TITLE.descending(),
7    ],
8  );
9  print('Posts: $posts');
10}

Pagination

Query results can also be paginated by passing in a page number (starting at 0) and an optional limit (defaults to 100). This will return a list of the first 100 items:

1// Do not forget to import the following with the other imports at the top of the file
2import 'package:amplify_datastore_plugin_interface/amplify_datastore_plugin_interface.dart';
3
4Future<void> fetchPostsWithPagination(int page) async {
5  final posts = await Amplify.DataStore.query(
6    Post.classType,
7    pagination: QueryPagination(page: page, limit: 25),
8  );
9  print('Posts: $posts');
10}

Query, then observe changes

To both query and observe subsequent changes to a Model, consider using observeQuery.