New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  init () {
    this._super(...arguments);
    this.set('parent', this.get('store').createRecord('parent'));
   },
  
  actions: {
    add() {
      let child = this.get('store').createRecord('child');
      this.get('parent.children').pushObject(child);
    },
    notify() {
      this.get('parent').notifyPropertyChange('children');
      console.log('notify');
    }
  }
});

models.child.js

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

export default Model.extend({
});

models.parent.js

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

export default Model.extend({
  children: hasMany('child')
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
<h3>parent: {{parent}}</h3>
<h3>Children: {{parent.children.length}}</h3>
<button onclick={{action 'add'}}>Add</button>
<button onclick={{action 'notify'}}>Notify</button>
{{outlet}}
<br>
<br>

twiddle.json

{
  "version": "0.15.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.2.2",
    "ember-template-compiler": "3.2.2",
    "ember-testing": "3.2.2"
  },
  "addons": {
    "ember-data": "3.2.0"
  }
}