each on many relationship

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName:'Ember Twiddle',
  actions: {
  	add() {
      let child = this.get('model');
      let quantity = child.get('quantity');
      quantity++;
      child.set('quantity', quantity);
    }
  }
});

application.route.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
  	let parent = this.store.createRecord('parent');
    return this.store.createRecord('child', {quantity: 3, parent});
  }
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<button {{action "add"}}>add</button>
<br>
child: {{model}}
<br>
child quantity: {{model.quantity}}
<br>
parent: {{model.parent}}
<br>
parent total: {{model.parent.total}}

models.child.js

import DS from 'ember-data';

export default DS.Model.extend({
  parent: DS.belongsTo('parent'),
  quantity: DS.attr('number'),
});

models.parent.js

import DS from 'ember-data';

export default DS.Model.extend({
  childs: DS.hasMany('child'),
  total: Ember.computed('childs.@each.quantity', {
    get() {
      return this.get('childs')
      	.map(c => c.get('quantity'))
        .reduce((t, c) => t += c, 0);
    }
  })
});

twiddle.json

{
  "version": "0.5.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js"
  }
}