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 sample schema:

enum PostStatus {
  ACTIVE
  INACTIVE
}

type Post @model {
  id: ID!
  title: String!
  rating: Int!
  status: PostStatus!
  # New field with @hasMany
  comments: [Comment] @hasMany
}

# New model
type Comment @model {
  id: ID!
  post: Post @belongsTo
  content: String!
}

Saving 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:

const post = await DataStore.save(
  new Post({
    title: 'My Post with comments',
    rating: 10,
    status: PostStatus.ACTIVE
  })
);

await DataStore.save(
  new Comment({
    content: 'Loving Amplify DataStore!',
    post: post
  })
);

Querying relations

You can query related models in three ways:

• Option 1: use the toArray() function to lazy load related comments

const post = await DataStore.query(Post, "YOUR_POST_ID")
if (post) {
  const comments = await post.comments.toArray()
}

const post = await DataStore.query(Post, "YOUR_POST_ID");
const comments = await post.comments.toArray();

• Option 2: use async iterators to lazy load related comments

const post = await DataStore.query(Post, "YOUR_POST_ID");
if (post) {
  for await (const comment of post.comments) {
    console.log(comment)
  };
}

const post = await DataStore.query(Post, "YOUR_POST_ID");
for await (const comment of post.comments) {
  console.log(comment)
};

• Option 3: use nested query predicates with the Comment model

const comments = await DataStore.query(Comment, c => c.post.id.eq('YOUR_POST_ID'));

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 synced to cloud. For example, the following operation would remove the Post with id 123 as well as any related comments:

const toDelete = await DataStore.query(Post, "123");
if (toDelete) {
  DataStore.delete(toDelete);
}

const toDelete = await DataStore.query(Post, "123");
DataStore.delete(toDelete);

However, in a many-to-many relationship the children are not removed and you must explicitly delete them.

Many-to-many relationships

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")
}

// first you save the post
const post = await DataStore.save(
  new Post({
    title: 'My first post'
  })
);

// secondly, you save the editor/user
const editor = await DataStore.save(
  new User({
    username: 'Nadia'
  })
);

// then you save the model that links a post with an editor
await DataStore.save(
  new PostEditor({
    post: post,
    user: editor
  })
);

Create

In order to save a many-to-many relationship, create both model instance first and then save them to a new join model instance.

const post = await DataStore.save(
  new Post({
    title: 'Amplify Weekly',
    rating: 10,
    status: PostStatus.ACTIVE
  })
);

const user = await DataStore.save(
  new User({
    username: 'rene'
  })
);

const postEditor = await DataStore.save(
  new PostEditor({
    post: post,
    user: user
  })
);

Here we've first created a new Post instance and a new User instance. Then, saved those instances to a new instance of our join model PostEditor.

Query

To query many-to-many relationships, use a nested predicate to filter the target model by the source model's id:

const editors = await DataStore.query(User, (u) => u.posts.post.id.eq(post.id));

Delete

Deleting the join model instance will not delete any source model instances.

await DataStore.delete(toBeDeletedPostEditor);

Both the Post and the User instances will not be deleted. Only the join model instances containing the link between a Post and a User.

Deleting a source model instance will also delete the join model instances containing the source model instance.

await DataStore.delete(toBeDeletedUser);

The toBeDeletedUser User instance and all PostEditor instances where user is linked to toBeDeletedUser will be deleted.