title: App Engine Django-Nonrel ReferenceProperty
body: <p>I am in the process of upgrading an App Engine Python/Django application from the (old style) App Engine Helper to the (new style) Django-Nonrel. Each model now inherits from <code>django.db.models.Model</code> instead of (old style) <code>appengine_django.models.BaseModel</code>.</p>
  
  <p>My database models are declared as follows:</p>
  
  <pre><code>from django.db import models
  
  class UserModelBackupTracker(models.Model):
  ...
  
  class UserModel(models.Model):
      # The following line generates the error
      backup_tracker = db.ReferenceProperty(reference_class=UserModelBackupTracker)
  </code></pre>
  
  <p>Which triggers the error:</p>
  
  <pre><code>"reference_class must be Model or _SELF_REFERENCE" (in appengine.ext.db.\__init__.py -&gt; ReferenceProperty.\__init__ ).
  </code></pre>
  
  <p>This error is triggered by a check to see that the reference_class "issubclass" of <code>google.appengine.ext.db.Model</code>, however the declaration of that class is of type <code>django.db.models.Model</code>, which is <em>not</em> a subclass of <code>ext.db.Model</code>.</p>
  
  <p>So, the question I have is: What am I doing wrong with respect to declaring the ReferenceProperty when using Django-Nonrel, and how should I fix it.</p>
  
  <p>Kind Regards
  Alexander</p>