title: KeyError when trying to save model instance. Django
body: <p>KeyError when trying to save model instance.
  It has to react on post_save signal than save instance...</p>
  
  <p>Code:</p>
  
  <pre><code>from django.db.models.signals import post_save
  
  class PlaylistEntry(models.Model):
      playlist=models.ForeignKey(Playlist)
      media=models.ForeignKey(Media)
      order=models.PositiveIntegerField(default=9000000, editable=False)
      added=models.DateTimeField(default=datetime.datetime.now(),editable=False )
  
  def playlist_entry_changed(sender, instance, **kwargs):
          entrys=PlaylistEntry.objects.filter(playlist=instance.playlist).order_by('order')
          entrys[0].save()
  
  post_save.connect(playlist_entry_changed, PlaylistEntry)
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>Exception Type: KeyError at /admin/playlist/playlistentry/add/
  Exception Value: 38539456
  </code></pre>