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controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  store: Ember.inject.service(),

  init: function() {
    this.get('store').pushPayload({
      folder: {
        id: 1,
        name: 'My folder'
      }
    });

    this.get('store').pushPayload({
      item: {
        id: 2,
        name: 'My item',
        parentFolder: 1
      }
    });

    var folder = this.get('store').peekRecord('folder', 1);
    var item = this.get('store').peekRecord('item', 2);

    console.log('Parent folder id: ', item.get('parentFolder.id'), 'Items length', folder.get('items.length'));
    item.get('parentFolder').get('items').removeObject(item);
    console.log('Parent folder id: ', item.get('parentFolder.id'), 'Items length', folder.get('items.length'));

    this.get('store').pushPayload({
      item: {
        id: 2,
        name: 'My item',
        parentFolder: 1
      }
    });
    console.log('Parent folder id: ', item.get('parentFolder.id'), 'Items length', folder.get('items.length'));

    this.set('folder', this.get('store').peekRecord('folder', 1));

    this._super.apply(this, arguments);
  }
});

models.folder.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { belongsTo, hasMany } from "ember-data/relationships";

export default Model.extend({
  name: attr('string'),
  items: hasMany('items', { inverse: 'parentFolder'})
});

models.item.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { belongsTo, hasMany } from "ember-data/relationships";

export default Model.extend({
  name: attr('string'),
  parentFolder: belongsTo('folder', { inverse: 'items' })
});

serializers.application.js

import DS from 'ember-data';

export default DS.RESTSerializer.extend({
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
items length {{folder.items.length}}
<br>
<br>

twiddle.json

{
  "version": "0.13.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.16.2",
    "ember-template-compiler": "2.16.2",
    "ember-testing": "2.16.2"
  },
  "addons": {
    "ember-data": "2.16.3"
  }
}