'Notice when the forum field changes.
A property won\'t do here, because it usurps the "forum" name and
prevents us from using lookups like Thread.objects.filter(forum=f).'

def __setattr__(self, attr, val):

    if ((attr == 'forum') and (not hasattr(self, '_old_forum'))):
        try:
            old = self.forum
        except AttributeError:
            pass
        else:
            self._old_forum = old
    super(Thread, self).__setattr__(attr, val)
