BelongsTo example and question

cake.hbs

<select onchange={{action (mut cake.factoryId) value="target.value"}} class="form-control">
  <option>-- Choose one --</option>
  {{#each factories as |factory|}}
    <option value={{factory.id}} selected={{eq factory.id cake.factoryId}}>{{factory.name}}</option>
  {{/each}}
</select>
<button type="submit" class="btn btn-default" {{action 'saveCake' cake}}">Save</button>

cake.js

//app/models/cake.js
import DS from 'ember-data';
import Ember from 'ember';

export default DS.Model.extend({
  name: DS.attr('string'),
  factory: DS.belongsTo('factory', { async: true }),
  factoryId: Ember.computed('factory', function() {
    return this.get('factory.id');
  }),
}),

edit.js

actions: {
  saveCake(cake) {
    this.store.findRecord('factory', cake.factoryId).then((factory) => {
      cake.set('factory', factory);
      cake.save().then(() => this.transitionTo('admin.cake'),
      (error) => {
        Ember.Logger.debug(error);
      });
    });
  },
}