For Wagner - hasMany/belongsTo in Ember w/ model going the other direction

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

models.billing-period.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(),
  invoices: hasMany('invoice')
});

models.invoice.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(),
  billingPeriod: belongsTo('billingPeriod')
});

routes.index.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    let bp1 = this.store.createRecord('billingPeriod', { name: 'Test Period' });
    bp1.save();
    
    let inv1 = this.store.createRecord('invoice', { name: 'Test Invoice Name for inv1', billingPeriod: bp1});
    inv1.save();
    let inv2 = this.store.createRecord('invoice', { name: 'Test Invoice Name for inv2', billingPeriod: bp1});
    inv2.save();
    
    // return bp1;
    return [inv1, inv2];
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

templates.index.hbs

{{!-- Billing Period Name: {{model.name}}
<br>
Invoices:
{{#each model.invoices as |invoice|}}
  <li>{{invoice.name}}</li>
{{/each}} --}}

<h3>Invoices:</h3>
<ul>
	{{#each model as |invoice|}}
		<li>
    	{{invoice.name}}
      {{#if invoice.billingPeriod }}
      	<ul>
        	<li>
          	<strong>Billing Period:</strong> {{invoice.billingPeriod.name}}
          </li>
        </ul>
      {{/if}}
    </li>
	{{/each}}
</ul>

twiddle.json

{
  "version": "0.8.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.5.1",
    "ember-data": "2.5.2",
    "ember-template-compiler": "2.5.1"
  }
}