title: Ember.js - Error - "Assertion failed: You must include an `id` in a hash passed to `push`"
body: <p>I'm getting this error after I save a post (title, text) to a mongodb database via a REST API written with Express and refresh the browser. I've already set my primary key to '_id' and have been reading about possibly normalizing the data? </p>
  
  <p>Here is the payload from the server (only 1 post in db):</p>
  
  <pre class="lang-js prettyprint-override"><code>{
    "posts": [
    {
      "title": "The Title",
      "text": "Lorem ipsum",
      "_id": "52c22892381e452d1d000010",
      "__v": 0
     }
    ]
  }
  </code></pre>
  
  <p>The controller:</p>
  
  <pre><code>App.PostsController = Ember.ArrayController.extend({
      actions: {
        createPost: function() {
          // Dummy content for now
          var to_post = this.store.createRecord('post', {
            title: 'The Title',
            text: 'Lorem ipsum'
          });
          to_post.save();
        }
      } 
   });
  </code></pre>
  
  <p>The model:</p>
  
  <pre><code>App.Post = DS.Model.extend({
     title: DS.attr('string'),
     text: DS.attr('string')
  });
  </code></pre>
  
  <p>Serializer:</p>
  
  <pre><code>App.MySerializer = DS.RESTSerializer.extend({
    primaryKey: function(type){
      return '_id';
    }
  });
  </code></pre>
  
  <p>Adapter:</p>
  
  <pre><code>App.ApplicationAdapter = DS.RESTAdapter.extend({
    namespace: 'api'
  });
  </code></pre>
  
  <p>Any help is much appreciated! Please let me know if you need any other info.
  Thanks</p>