Relationships

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

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

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:

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

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

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

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

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 }.

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')
});

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:

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:

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:

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:

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

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);
});

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.