title: Ember data : createObject with an initial async relationship value
body: <p>I've been using ember and ember data for a few months and I am now working on optimisations of my webapp. </p>
  
  <p>I have this <code>feed</code> model :</p>
  
  <pre><code>export default DS.Model.extend({
    app: DS.belongsTo('app', {async:true}),
    icon: DS.attr('string'),
    title: DS.attr('string'),
    typeName: DS.attr('string'),
    rootElement: DS.belongsTo('element', {async:true}),
    position: DS.attr('number'),
    fontAwesomeIcon:DS.attr('string'),
  
    htmlId:function () {
      return "module_"+this.get('id');
    }.property(),
  });
  </code></pre>
  
  <p>When I am trying to create a new <code>feed</code> object I only need to specify my backend an app and a type so I do this :</p>
  
  <pre><code>addFeed: function() {
      var app=this.get('model');
      var selectTypeForm = document.getElementById('selectFeedType');
      var type = selectTypeForm.options[selectTypeForm.selectedIndex].value
      var newFeed = this.store.createRecord('feed', {
          app:this.get('model'),
          typeName:
      });          
      newFeed.save();
  },
  </code></pre>
  
  <p>My problem is that if the <code>app</code> property is async (like in this example) the app id is never sent to my backend on the POST request (it's like I never set it). 
  If I switch it to <code>async:false</code> in the feed model, it accepts my app id at creation and correctly sends it to my backend.</p>
  
  <p>Is it impossible to write an async property ?</p>
  
  <p>Thanks</p>
  
  <p>Versions : <code>Ember 1.7.0</code> and <code>Ember data  1.0.0-beta.10</code></p>