title: ember-data: Allow sideloaded model to contain embedded records that were just created
body: <p>I’m running into an issue with the REST Adapter/Serializer and EmbeddedRecordsMixin, specifically my app needs to be able to sideload an model’s parent record in create/POST responses, and that parent model contains the child record embedded in a hasMany array. This is resulting in “You cannot update the id index of an InternalModel once set.” errors. heres a simplified version of the JSON structure my api is returning:</p>
  
  <pre><code>// POST /child-model response
  {
      "childModel": {
          "id": 1,
          "foo": "Bar"
      },
      "parentModel": {
          "id": 2,
          "children": [
              {
                  "id": 1,
                  "foo": "Bar"
              }
          ]
      }
  }
  </code></pre>
  
  <p>Is there any way to allow this type of response? Essentially I want ember-data to just treat the embedded child record as just an "update" to the record that was just created, where it is just using it to make the association between it and the parent record.</p>