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

    let team = Team(mantra: "Go Frontend!")
    let createdTeam = try await Amplify.API.mutate(request: .create(team)).get()
    let member = Member(
        name: "Tim",
        team: createdTeam) // Directly pass in the team instance
    let createdMember = try await Amplify.API.mutate(request: .create(member))
    print("Create team or member failed", error)

Update a "Has Many" relationship between records

    let newTeam = Team(mantra: "Go Fullstack!")
    let createdNewTeam = try await Amplify.API.mutate(request: .create(newTeam)).get()
    existingMember.setTeam(createdNewTeam)
    let updatedMember = try await Amplify.API.mutate(request: .update(existingMember)).get()
    print("Create team or update member failed", error)

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.

    existingMember.setTeam(nil)
    let memberRemovedTeam = try await Amplify.API.mutate(request: .update(existingMember)).get()
    print("Failed to remove team from member", error)

Lazy load a "Has Many" relationship

    let queriedTeam = try await Amplify.API.query(
        request: .get(
            Team.self,
            byIdentifier: team.identifier)).get()
    guard let queriedTeam, let members = queriedTeam.members else {
        print("Missing team or members")
        return
    try await members.fetch()
    print("Number of members: \(members.count)")
    print("Failed to fetch team or members", error)

Eagerly load a "Has Many" relationship

    let queriedTeamWithMembers = try await Amplify.API.query(
        request: .get(
            Team.self,
            byIdentifier: team.identifier,
            includes: { team in [team.members]}))
        .get()
    guard let queriedTeamWithMembers, let members = queriedTeamWithMembers.members else {
        print("Missing team or members")
        return
    print("Number of members: \(members.count)")
    print("Failed to fetch team with members", error)

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.

    let customer = Customer(name: "Rene")
    let createdCustomer = try await Amplify.API.mutate(request: .create(customer)).get()
    let cart = Cart(
        items: ["Tomato", "Ice", "Mint"],
        customer: createdCustomer)
    let createdCart = try await Amplify.API.mutate(request: .create(cart)).get()
    print("Create customer or cart failed", error)

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:

    let newCustomer = Customer(name: "Rene")
    let newCustomerCreated = try await Amplify.API.mutate(request: .create(newCustomer)).get()
    existingCart.setCustomer(newCustomerCreated)
    let updatedCart = try await Amplify.API.mutate(request: .update(existingCart)).get()
    print("Create customer or cart failed", error)

Delete a "Has One" relationship between records

You can set the relationship field to nil to delete a "Has One" relationship between records.

    existingCart.setCustomer(nil)
    let cartWithCustomerRemoved = try await Amplify.API.mutate(request: .update(existingCart)).get()
    print("Failed to remove customer from cart", error)

    guard let queriedCart = try await Amplify.API.query(
        request: .get(
            Cart.self,
            byIdentifier: existingCart.identifier)).get() else {
        print("Missing cart")
        return
    let customer = try await queriedCart.customer
    print("Failed to fetch cart or customer", error)

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:

    guard let queriedPost = try await Amplify.API.query(
        request: .get(
            Post.self,
            byIdentifier: post.identifier)).get() else {
        print("Missing post")
        return
    let loadedAuthor = try await queriedPost.author
    let loadedEditor = try await queriedPost.editor
    print("Failed to fetch post, author, or editor", error)

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