New Twiddle

controllers.application.js

import Ember from 'ember';

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

macros.belongs-to-with-default.js

import Ember from 'ember'
import RSVP from 'rsvp'

const ObjectPromiseProxy = Ember.ObjectProxy.extend(Ember.PromiseProxyMixin)

export default function(dependentKey, defaultValues={}) {
  return Ember.computed(dependentKey, {
    get() {
      const promise = this.get(dependentKey).then(model => model || RSVP.reject()).catch((e) => {
        const { type } = this.get('constructor.relationshipsByName').get(dependentKey)
        const model = this.get('store').createRecord(type, defaultValues)
        
        return this.set(dependentKey, model)
      })
      
      return ObjectPromiseProxy.create({ promise })
    },

    set(_, model) {
      return this.set(dependentKey, model)
    }
  })
}

models.credit-card.js

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

export default Model.extend({
  number: attr('string'),
  customer: belongsTo()
});

models.customer.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { belongsTo, hasMany } from "ember-data/relationships";
import belongsToWithDefault from '../macros/belongs-to-with-default';

export default Model.extend({
  name: attr('string'),
  creditCard: belongsTo(),
  creditCardWithDefault: belongsToWithDefault('creditCard', {
    number: '123456'
  })
});

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    return this.store.createRecord('customer', {
      name: 'fooCorp'
    }).get('creditCardWithDefault');
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{model.number}}
{{model.customer.name}}
<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"
  }
}