Modeling relationships

When modeling application data, you often need to establish relationships between different data models. In Amplify Data, you can create one-to-many, one-to-one, and many-to-many relationships in your Data schema. On the client-side, Amplify Data allows you to lazy or eager load of related data.

With Amplify Data Construct @aws-amplify/data-construct@1.8.4, an improvement was made to how relational field data is handled in subscriptions when different authorization rules apply to related models in a schema. The improvement redacts the values for the relational fields, displaying them as null or empty, to prevent unauthorized access to relational data.

This redaction occurs whenever it cannot be determined that the child model will be protected by the same permissions as the parent model.

Because subscriptions are tied to mutations and the selection set provided in the result of a mutation is then passed through to the subscription, relational fields in the result of mutations must be redacted.

If an authorized end-user needs access to the redacted relational fields, they should perform a query to read the relational data.

Additionally, subscriptions will inherit related authorization when relational fields are set as required. To better protect relational data, consider modifying the schema to use optional relational fields.

Types of relationships

Relationship	Code	Description	Example
one to many	`a.hasMany(...)` & `a.belongsTo(...)`	Creates a one-to-many relationship between two models.	A Team has many Members. A Member belongs to a Team.
one to one	`a.hasOne(...)` & `a.belongsTo(...)`	Creates a one-to-one relationship between two models.	A Customer has one Cart. A Cart belongs to one Customer.
many to many	Two `a.hasMany(...)` & `a.belongsTo(...)` on join tables	Create two one-to-many relationships between the related models in a join table.	A Post has many Tags. A Tag has many Posts.

Model one-to-many relationships

Create a one-to-many relationship between two models using the hasMany() and belongsTo() method. In the example below, a Team has many Members and a Member belongs to exactly one Team.

Create a reference field called teamId on the Member model. This reference field's type MUST match the type of Team's identifier. In this case, it's an auto-generated id: a.id().required() field.
Add a relationship field called team that references the teamId field. This allows you to query for the team information from the Member model.
Add a relationship field called members that references the teamId field on the Member model.

const schema = a.schema({
  Member: a.model({
    name: a.string().required(),
    // 1. Create a reference field
    teamId: a.id(),
    // 2. Create a belongsTo relationship with the reference field
    team: a.belongsTo('Team', 'teamId'),
  Team: a.model({
    mantra: a.string().required(),
    // 3. Create a hasMany relationship with the reference field
    //    from the `Member`s model.
    members: a.hasMany('Member', 'teamId'),
}).authorization((allow) => allow.publicApiKey());

Create a "Has Many" relationship between records

final team = Team(mantra: "Go Frontend!");
final teamRequest = ModelMutations.create(team);
final teamResponse = await Amplify.API.mutate(request: teamRequest).response;
final member = Member(name: "Tim", team: teamResponse.data);
final memberRequest = ModelMutations.create(member);
final memberResponse = await Amplify.API.mutate(request: memberRequest).response;

Update a "Has Many" relationship between records

final newTeam = Team(mantra: "Go Fullstack!");
final newTeamRequest = ModelMutations.create(team);
final newTeamResponse = await Amplify.API.mutate(request: teamRequest).response;
final memberWithUpdatedTeam = existingMember.copyWith(team: newTeamResponse.data);
final memberUpdateRequest = ModelMutations.update(memberWithUpdatedTeam);
final memberUpdateResponse = await Amplify.API.mutate(request: memberUpdateRequest).response;

Delete a "Has Many" relationship between records

If your reference field is not required, then you can "delete" a one-to-many relationship by setting the relationship value to null.

final memberWithRemovedTeam = existingMember.copyWith(team: null);
final memberRemoveRequest = ModelMutations.update(memberWithRemovedTeam);
final memberRemoveResponse = await Amplify.API.mutate(request: memberRemoveRequest).response;

// Fetch the team with the team id.
final teamRequest = ModelQueries.get<Team>(
    Team.classType, TeamModelIdentifier(id: "YOUR_TEAM_ID"));
final teamResult = await Amplify.API.query(request: teamRequest).response;
final team = teamResult.data!;
// Define a limit for your pagination
const limit = 100;
// Do the initial call to get the initial items
final firstRequest = ModelQueries.list<Member>(Member.classType,
    limit: limit, where: Member.TEAMID.eq(team.id));
final firstResult = await Amplify.API.query(request: firstRequest).response;
final firstPageData = firstResult.data;
// If there are more than 100 items you can reiterate the following code to get next pages.
if (firstPageData?.hasNextResult ?? false) {
  final secondRequest = firstPageData!.requestForNextResult;
  final secondResult =
      await Amplify.API.query(request: secondRequest!).response;
  return secondResult.data?.items ?? <Member?>[];
  // You can return the page data by calling items property.
  return firstPageData?.items ?? <Member?>[];

Model a "one-to-one" relationship

Create a one-to-one relationship between two models using the hasOne() and belongsTo() methods. In the example below, a Customer has a Cart and a Cart belongs to a Customer.

Create a reference field called customerId on the Cart model. This reference field's type MUST match the type of Customer's identifier. In this case, it's an auto-generated id: a.id().required() field.
Add a relationship field called customer that references the customerId field. This allows you to query for the customer information from the Cart model.
Add a relationship field called activeCart that references the customerId field on the Cart model.

const schema = a.schema({
  Cart: a.model({
    items: a.string().required().array(),
    // 1. Create reference field
    customerId: a.id(),
    // 2. Create relationship field with the reference field
    customer: a.belongsTo('Customer', 'customerId'),
  Customer: a.model({
    name: a.string(),
    // 3. Create relationship field with the reference field
    //    from the Cart model
    activeCart: a.hasOne('Cart', 'customerId')
}).authorization((allow) => allow.publicApiKey());

Create a "Has One" relationship between records

To create a "has one" relationship between records, first create the parent item and then create the child item and assign the parent.

final customer = Customer(name: "Rene");
final customerRequest = ModelMutations.create(customer);
final customerResponse = await Amplify.API.mutate(request: customerRequest).response;
final cart = Cart(items: ["Tomato", "Ice", "Mint"], customer: teamResponse.customer);
final cartRequest = ModelMutations.create(cart);
final cartResponse = await Amplify.API.mutate(request: cartRequest).response;

Update a "Has One" relationship between records

To update a "Has One" relationship between records, you first retrieve the child item and then update the reference to the parent to another parent. For example, to reassign a Cart to another Customer:

final newCustomer = Customer(name: "Ian");
final newCustomerRequest = ModelMutations.create(newCustomer);
final newCustomerResponse = await Amplify.API.mutate(request: newCustomerRequest).response;
final cartWithUpdatedCustomer = existingCart.copyWith(customer: newCustomerResponse.data);
final cartUpdateRequest = ModelMutations.update(cartWithUpdatedCustomer);
final cartUpdateResponse = await Amplify.API.mutate(request: cartUpdateRequest).response;

Delete a "Has One" relationship between records

final cartWithRemovedCustomer = existingCart.copyWith(customer: null);
final cartRemoveRequest = ModelMutations.update(cartWithRemovedCustomer);
final cartRemoveResponse = await Amplify.API.mutate(request: cartRemoveRequest).response;

// Fetch the cart with the cart id.
final cartRequest = ModelQueries.get<Cart>(
    Cart.classType, CartModelIdentifier(id: "MY_CART_ID"));
final cartResult = await Amplify.API.query(request: cartRequest).response;
final cart = cartResult.data!;
// Do the customer call to with the id from cart
if (cart.customerId != null) {
  final customerRequest = ModelQueries.get<Customer>(
      Customer.classType, CustomerModelIdentifier(id: cart.customerId!));
  final customerResult =
      await Amplify.API.query(request: customerRequest).response;
  final customer = customerResult.data!;

Model a "many-to-many" relationship

In order to create a many-to-many relationship between two models, you have to create a model that serves as a "join table". This "join table" should contain two one-to-many relationships between the two related entities. For example, to model a Post that has many Tags and a Tag has many Posts, you'll need to create a new PostTag model that represents the relationship between these two entities.

const schema = a.schema({
  PostTag: a.model({
    // 1. Create reference fields to both ends of
    //    the many-to-many relationship
    postId: a.id().required(),
    tagId: a.id().required(),
    // 2. Create relationship fields to both ends of
    //    the many-to-many relationship using their
    //    respective reference fields
    post: a.belongsTo('Post', 'postId'),
    tag: a.belongsTo('Tag', 'tagId'),
  Post: a.model({
    title: a.string(),
    content: a.string(),
    // 3. Add relationship field to the join model
    //    with the reference of `postId`
    tags: a.hasMany('PostTag', 'postId'),
  Tag: a.model({
    name: a.string(),
    // 4. Add relationship field to the join model
    //    with the reference of `tagId`
    posts: a.hasMany('PostTag', 'tagId'),
}).authorization((allow) => allow.publicApiKey());

Model multiple relationships between two models

Relationships are defined uniquely by their reference fields. For example, a Post can have separate relationships with a Person model for author and editor.

const schema = a.schema({
  Post: a.model({
    title: a.string().required(),
    content: a.string().required(),
    authorId: a.id(),
    author: a.belongsTo('Person', 'authorId'),
    editorId: a.id(),
    editor: a.belongsTo('Person', 'editorId'),
  Person: a.model({
    name: a.string(),
    editedPosts: a.hasMany('Post', 'editorId'),
    authoredPosts: a.hasMany('Post', 'authorId'),
}).authorization((allow) => allow.publicApiKey());

On the client-side, you can fetch the related data with the following code:

const client = generateClient<Schema>();
const { data: post } = await client.models.Post.get({ id: "SOME_POST_ID" });
const { data: author } = await post?.author();
const { data: editor } = await post?.editor();

Model relationships for models with sort keys in their identifier

In cases where your data model uses sort keys in the identifier, you need to also add reference fields and store the sort key fields in the related data model:

const schema = a.schema({
  Post: a.model({
    title: a.string().required(),
    content: a.string().required(),
    // Reference fields must correspond to identifier fields.
    authorName: a.string(),
    authorDoB: a.date(),
    // Must pass references in the same order as identifiers.
    author: a.belongsTo('Person', ['authorName', 'authorDoB']),
  Person: a.model({
    name: a.string().required(),
    dateOfBirth: a.date().required(),
    // Must reference all reference fields corresponding to the
    // identifier of this model.
    authoredPosts: a.hasMany('Post', ['authorName', 'authorDoB']),
  }).identifier(['name', 'dateOfBirth']),
}).authorization((allow) => allow.publicApiKey());

Make relationships required or optional

Amplify Data's relationships use reference fields to determine if a relationship is required or not. If you mark a reference field as required, then you can't "delete" a relationship between two models. You'd have to delete the related record as a whole.

const schema = a.schema({
  Post: a.model({
    title: a.string().required(),
    content: a.string().required(),
    // You must supply an author when creating the post
    // Author can't be set to `null`.
    authorId: a.id().required(),
    author: a.belongsTo('Person', 'authorId'),
    // You can optionally supply an editor when creating the post.
    // Editor can also be set to `null`.
    editorId: a.id(),
    editor: a.belongsTo('Person', 'editorId'),
  Person: a.model({
    name: a.string(),
    editedPosts: a.hasMany('Post', 'editorId'),
    authoredPosts: a.hasMany('Post', 'authorId'),
}).authorization((allow) => allow.publicApiKey());

Add fields to data model

Customize data model identifiers