New Twiddle

adapters.application.js

import DS from 'ember-data';

export default DS.JSONAPIAdapter.extend({
  findRecord() {
    return {
      data: {
        type: 'parent-object',
        id: '2',
        attributes: {
          name: 'foo'
        }
      }
    };
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  actions: {
    foo() {
      this.store.findRecord('parent-object', 2).then(parent => {
        const child = this.store.createRecord('child-object');
        parent.get('children').pushObject(child);
        alert(child.get('parent.name'));
      });
    },
    foo2() {
      this.store.findRecord('parent-object', 2).then(parent => {
        const child = this.store.createRecord('child-object');
        child.set('parent', parent);
        
        alert(parent.get('children.length'));
      });
    }
  }
});

models.child-object.js

import DS from 'ember-data';

export default DS.Model.extend({
    name: DS.attr('string'),
    parent: DS.belongsTo('parent-object')
});

models.parent-object.js

import DS from 'ember-data';

export default DS.Model.extend({
    name: DS.attr('string'),
    children: DS.hasMany('child-object')
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
<button onclick={{action 'foo'}}>foo</button>

<button onclick={{action 'foo2'}}>foo2</button>
<br>
<br>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}