title: Django GenericRelated field conditional Query raising 'GenericRelation' object has no attribute 'field'
body: <p>I have an event object, there are other objects besides <code>Notes</code> that have a generic relation to <code>Event</code>, and which don't have an active field. Now I want to be able to write a query which excludes all <code>Events</code> where the active field of the <code>Notes</code> is False. So I tried doing the following.</p>
  
  <pre><code>queryset = Event.objects.all()
  filters = (
          Q(content_type__model='notes') &amp;
          Q(note__active=False)
      )
  queryset = queryset.exclude(filters)
  </code></pre>
  
  <p>This didn't work because it's running the query separately, and when it tries to execute for items for which there are no content_object or which are not of type <code>Notes</code>, it fails and gives the following error:</p>
  
  <pre><code>AttributeError 'GenericRelation' object has no attribute 'field'.
  </code></pre>
  
  <hr>
  
  <pre><code>class Event(models.Model):
      content_type = models.ForeignKey(ContentType, null=True, blank=True)
      object_id = models.PositiveIntegerField(null=True, blank=True)
      content_object = GenericForeignKey('content_type', 'object_id')
  
  class Notes(models.Model):
      active = models.BooleanField(default=True)
      event_log = GenericRelation(
          'Event',
          related_query_name='note'
      )
  </code></pre>