http://thejsguy.com/2017/02/20/encapsulate-relationship-logic-in-your-ember-data-model.html

post.js

// app/models/post.js
export default DS.Model.extend({
  body: DS.attr(),
  comments: DS.hasMany(),
  addComment(commentData) {
    let store = Ember.getOwner(this).lookup('service:store');
    let comment = store.createRecord('comment', commentData);
    this.get('comments').pushObject(comment);
    return comment.save().then(() => {
      return this.save();
    });
  }
});