Store a string representation of content_object as target
        and actor name for fast retrieval and sorting.

def save(self, *args, **kwargs):
        """
        Store a string representation of content_object as target
        and actor name for fast retrieval and sorting.
        """
        if not self.target:
            self.target = str(self.content_object)
        if not self.actor_name:
            self.actor_name = str(self.actor)
        super(Activity, self).save()