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 @auth(rules: [{allow: public}]) { id: ID! title: String! rating: Int! status: PostStatus! # new field with @hasMany comments: [Comment] @hasMany}
# new modeltype Comment @model { id: ID! content: String post: Post @belongsTo}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:
let post = Post( title: "My post with comments", rating: 5, status: .active)
let commentWithPost = Comment( content: "Loving Amplify DataStore", post: post)
do { let savedPost = try await Amplify.DataStore.save(post) let savedCommentWithPost = try await Amplify.DataStore.save(commentWithPost)} catch let error as DataStoreError { print("Failed with error \(error)")} catch { print("Unexpected error \(error)")}let post = Post( title: "My post with comments", rating: 5, status: .active)
let commentWithPost = Comment( content: "Loving Amplify DataStore", post: post)
let sink = Amplify.Publisher.create { try await Amplify.DataStore.save(post) } .flatMap { Amplify.Publisher.create { try await Amplify.DataStore.save(commentWithPost) } } .sink { if case let .failure(error) = $0 { print("Error adding post and comment - \(error.localizedDescription)") } } receiveValue: { print("Post and comment saved!") }Querying relations
Models with one-to-many connections are lazy-loaded when accessing the connected property, so accessing a relation is as simple as:
do { guard let queriedPost = try await Amplify.DataStore.query(Post.self, byId: "123"), let comments = queriedPost.comments else { return } // call fetch to lazy load the postResult before accessing its result try await comments.fetch() for comment in comments { print("\(comment)") }} catch let error as DataStoreError { print("Failed to query \(error)")} catch let error as CoreError { print("Failed to fetch \(error)")} catch { print("Unexpected error \(error)")}let sink = Amplify.Publisher.create { try await Amplify.DataStore.query(Post.self, byId: "123") }.sink { if case let .failure(error) = $0 { print("Error retrieving post \(error.localizedDescription)") }} receiveValue: { queriedPost in guard let queriedPost = queriedPost, let comments = queriedPost.comments else { return } // call fetch to lazy load the postResult before accessing its result Task { do { try await comments.fetch() for comment in comments { print("\(comment)") } } catch let error as CoreError { print("Failed to fetch \(error)") } catch { print("Unexpected error \(error)") } }}The connected properties are of type List<M>, where M is the model type, and that type is a custom Swift Collection, which means that you can filter, map, etc:
let excitedComments = comments .compactMap { $0.content } .filter { $0.contains("Wow!") }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:
do { guard let postWithComments = try await Amplify.DataStore.query(Post.self, byId: "123") else { print("No post found") return } try await Amplify.DataStore.delete(postWithComments) print("Post with id 123 deleted with success")} catch let error as DataStoreError { print("Failed with error \(error)")} catch { print("Unexpected error \(error)")}let sink = Amplify.Publisher.create { guard let postWithComments = try await Amplify.DataStore.query(Post.self, byId: "123") else { return } try await Amplify.DataStore.delete(postWithComments)}.sink { if case let .failure(error) = $0 { print("Error deleting post and comments - \(error)") }} receiveValue: { print("Post with id 123 deleted with success")}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")}let post = Post( title: "My first post", status: .active)let user = User( username: "Nadia")let postEditor = PostEditor( post: post, user: user)do { try await Amplify.DataStore.save(post) try await Amplify.DataStore.save(user) try await Amplify.DataStore.save(postEditor) print("Saved post, user, and postEditor!")} catch let error as DataStoreError { print("Failed with error \(error)")} catch { print("Unexpected error \(error)")}let post = Post( title: "My first post", status: .active)let user = User( username: "Nadia")let postEditor = PostEditor( post: post, user: user)let sink = Amplify.Publisher.create{ try await Amplify.DataStore.save(post) } .flatMap { _ in Amplify.Publisher.create { try await Amplify.DataStore.save(user) } } .flatMap { _ in Amplify.Publisher.create { try await Amplify.DataStore.save(postEditor) } } .sink { if case let .failure(error) = $0 { print("Error saving post, user and postEditor: \(error.localizedDescription)") } } receiveValue: { _ in print("Saved user, post and postEditor!") }