'if Favorite exists for input user and type and pk of the input
content_object, return it.  else return None.
impl note: can only be 0 or 1, per the class\'s unique_together.'

def favorite_for_user_and_content_object(self, user, content_object):

    content_type = ContentType.objects.get_for_model(type(content_object))
    result = self.filter(user=user, content_type=content_type, object_id=content_object.pk)
    if (len(result) > 0):
        return result[0]
    else:
        return None
