Relationships

To declare a one-to-one relationship between two models, use DS.belongsTo:

import DS from 'ember-data';

export default DS.Model.extend({
  profile: DS.belongsTo('profile')
});

import DS from 'ember-data';

export default DS.Model.extend({
  user: DS.belongsTo('user')
});

To declare a one-to-many relationship between two models, use DS.belongsTo in combination with DS.hasMany, like this:

import DS from 'ember-data';

export default DS.Model.extend({
  comments: DS.hasMany('comment')
});

import DS from 'ember-data';

export default DS.Model.extend({
  blogPost: DS.belongsTo('blog-post')
});

To declare a many-to-many relationship between two models, use DS.hasMany:

import DS from 'ember-data';

export default DS.Model.extend({
  tags: DS.hasMany('tag')
});

import DS from 'ember-data';

export default DS.Model.extend({
  blogPosts: DS.hasMany('blog-post')
});

Ember Data will do its best to discover which relationships map to one another. In the one-to-many code above, for example, Ember Data can figure out that changing the comments relationship should update the blogPost relationship on the inverse because blogPost is the only relationship to that model.

However, sometimes you may have multiple belongsTo/hasManys for the same type. You can specify which property on the related model is the inverse using DS.belongsTo or DS.hasMany's inverse option. Relationships without an inverse can be indicated as such by including { inverse: null }.

import DS from 'ember-data';

export default DS.Model.extend({
  onePost: DS.belongsTo('blog-post', { inverse: null }),
  twoPost: DS.belongsTo('blog-post'),
  redPost: DS.belongsTo('blog-post'),
  bluePost: DS.belongsTo('blog-post')
});

import DS from 'ember-data';

export default DS.Model.extend({
  comments: DS.hasMany('comment', {
    inverse: 'redPost'
  })
});

When you want to define a reflexive relation (a model that has a relationship to itself), you must explicitly define the inverse relationship. If there is no inverse relationship then you can set the inverse to null.

Here's an example of a one-to-many reflexive relationship:

import DS from 'ember-data';

export default DS.Model.extend({
  children: DS.hasMany('folder', { inverse: 'parent' }),
  parent: DS.belongsTo('folder', { inverse: 'children' })
});

Here's an example of a one-to-one reflexive relationship:

import DS from 'ember-data';

export default DS.Model.extend({
  name: DS.attr('string'),
  bestFriend: DS.belongsTo('user', { inverse: 'bestFriend' }),
});

You can also define a reflexive relationship that doesn't have an inverse:

import DS from 'ember-data';

export default DS.Model.extend({
  parent: DS.belongsTo('folder', { inverse: null })
});

Some models may have properties that are deeply nested objects of readonly data. The naïve solution would be to define models for each nested object and use hasMany and belongsTo to recreate the nested relationship. However, since readonly data will never need to be updated and saved this often results in the creation of a great deal of code for very little benefit. An alternate approach is to define these relationships using an attribute with no transform (DS.attr()). This makes it easy to access readonly values in computed properties and templates without the overhead of defining extraneous models.

Let's assume that we have a blog-post and a comment model, which are related to each other as follows:

import DS from 'ember-data';

export default DS.Model.extend({
  comments: DS.hasMany('comment')
});

import DS from 'ember-data';

export default DS.Model.extend({
  blogPost: DS.belongsTo('blog-post')
});

When a user comments on a blogPost, we need to create a relationship between the two records. We can simply set the belongsTo relationship in our new comment:

let blogPost = this.get('store').peekRecord('blog-post', 1);
let comment = this.get('store').createRecord('comment', {
  blogPost: blogPost
});
comment.save();

This will create a new comment record and save it to the server. Ember Data will also update the blogPost to include our newly created comment in its comments relationship.

We could have also linked the two records together by updating the blogPost's hasMany relationship:

let blogPost = this.get('store').peekRecord('blog-post', 1);
let comment = this.get('store').createRecord('comment', {
});
blogPost.get('comments').pushObject(comment);
comment.save().then(function () {
  blogPost.save();
});

In this case the new comment's belongsTo relationship will be set to the parent blogPost.

Although createRecord is fairly straightforward, the only thing to watch out for is that you cannot assign a promise as a relationship, currently.

For example, if you want to set the author property of a blogPost, this would not work if the user with id isn't already loaded into the store:

this.get('store').createRecord('blog-post', {
  title: 'Rails is Omakase',
  body: 'Lorem ipsum',
  author: this.get('store').findRecord('user', 1)
});

However, you can easily set the relationship after the promise has fulfilled:

let blogPost = this.get('store').createRecord('blog-post', {
  title: 'Rails is Omakase',
  body: 'Lorem ipsum'
});

this.get('store').findRecord('user', 1).then(function(user) {
  blogPost.set('author', user);
});

When you request data from the server for a model that has relationships with one or more others, you may want to retrieve records corresponding to those related models at the same time. For example, when retrieving a blog post, you may need to access the comments associated with the post as well. The JSON API specification allows servers to accept a query parameter with the key include as a request to include those related records in the response returned to the client. The value of the parameter should be a comma-separated list of names of the relationships required.

If you are using an adapter that supports JSON API, such as Ember's default JSONAPIAdapter, you can easily add the include parameter to the server requests created by the findRecord(), findAll(), query() and queryRecord() methods.

findRecord() and findAll() each take an options argument in which you can specify the include parameter. For example, given a post model that has a hasMany relationship with a comment model, when retrieving a specific post we can have the server also return that post's comments as follows:

export default Ember.Route.extend({
  model(params) {
   return this.store.findRecord('post', params.post_id, {include: 'comments'});
  }
});

The post's comments would then be available in your template as model.comments.

Nested relationships can be specified in the include parameter as a dot-separated sequence of relationship names. So to request both the post's comments and the authors of those comments the request would look like this:

export default Ember.Route.extend({
  model(params) {
   return this.store.findRecord('post', params.post_id, {include: 'comments,comments.author'});
  }
});

The query() and queryRecord() methods each take a query argument that is serialized directly into the URL query string and the include parameter may form part of that argument. For example:

export default Ember.Route.extend({
  model() {
    // GET to /artists?filter[name]=Adele&include=albums
    this.store.query('artist', {
      filter: {name: 'Adele'},
      include: 'albums'
    }).then(function(artists) {
      return artists.get('firstObject');
    });
  }
});

Sometimes we want to set relationships on already existing records. We can simply set a belongsTo relationship:

let blogPost = this.get('store').peekRecord('blog-post', 1);
let comment = this.get('store').peekRecord('comment', 1);
comment.set('blogPost', blogPost);
comment.save();

Alternatively, we could update the hasMany relationship by pushing a record into the relationship:

let blogPost = this.get('store').peekRecord('blog-post', 1);
let comment = this.get('store').peekRecord('comment', 1);
blogPost.get('comments').pushObject(comment);
blogPost.save();

To remove a belongsTo relationship, we can set it to null, which will also remove it from the hasMany side:

let comment = this.get('store').peekRecord('comment', 1);
comment.set('blogPost', null);
comment.save();

It is also possible to remove a record from a hasMany relationship:

let blogPost = this.get('store').peekRecord('blog-post', 1);
let comment = this.get('store').peekRecord('comment', 1);
blogPost.get('comments').removeObject(comment);
blogPost.save();

As in the earlier examples, the comment's belongsTo relationship will also be cleared by Ember Data.

While working with relationships it is important to remember that they return promises.

For example, if we were to work on a blogPost's asynchronous comments, we would have to wait until the promise has fulfilled:

let blogPost = this.get('store').peekRecord('blog-post', 1);

blogPost.get('comments').then((comments) => {
  // now we can work with the comments
});

The same applies to belongsTo relationships:

let comment = this.get('store').peekRecord('comment', 1);

comment.get('blogPost').then((blogPost) => {
  // the blogPost is available here
});

Handlebars templates will automatically be updated to reflect a resolved promise. We can display a list of comments in a blogPost like so:

<ul>
  {{#each blogPost.comments as |comment|}}
    <li>{{comment.id}}</li>
  {{/each}}
</ul>

Ember Data will query the server for the appropriate records and re-render the template once the data is received.