title: Query proformance on GenericForeignKey Django
body: <p>I have optimized the query below the best I can.</p>
  
  <pre><code> message = Message.objects.defer('gateway', 'batch', 'content_type', 'sender',
                                              'reply_callback')\
                  .select_related().get(pk=message_id)
  </code></pre>
  
  <p>However, the model has a field called <code>billee</code> (see below) </p>
  
  <pre><code>billee = generic.GenericForeignKey()
  </code></pre>
  
  <p>I don't seem to be able to use select_related or defer on this field, maybe because its a GenericForeignKey. Can someone explain why and then give me an example of how to achieve this?  </p>