title: Ember data: save loses belongsTo relationship
body: <p>I have the following problem:</p>
  
  <p>A form with a select field for selecting the category of a post.  Let's say the post has category 100.  In Ember inspector, this is shown as follows:</p>
  
  <pre><code>category: &lt;App.Category:ember708:100&gt;
  </code></pre>
  
  <p>When I save the post (via Ember Data 1.0.0 beta 2), the category suddenly changes to:</p>
  
  <pre><code>category: 100
  </code></pre>
  
  <p>And the value is no longer selected in the select list.  It is cleared.</p>
  
  <p>Code to save:</p>
  
  <pre><code>    post.save().then(
              function () {
                alert("Save OK");
              }
      )
  </code></pre>
  
  <p>Any idea in which direction I need to search ...  If I transition to another page and then go back to the edit screen, the values are all correct. The data is thus still correct in the model ...</p>