title: django MPTT model and applying Fixture
body: <p>I'm using django MPTT model.</p>
  
  <p>I subclass MPTT model, but then try to add fixture to the custom model with supplied initial_data in JSON.</p>
  
  <p>The parent TreeForeignKey is optional (blank=True, null=True)</p>
  
  <p>When I apply a JSON fixture from initial_data, it asks to supply the fields "lft", "rght", "tree_id", "level".</p>
  
  <pre><code>ie: may not be NULL
  </code></pre>
  
  <p>...when running python manage.py syncdb</p>
  
  <p>These are fields from MPTT.</p>
  
  <p>Is there a way to exclude this or get around this from the fixture data?</p>
  
  <p>Thanks </p>