code: def new_reply(cls, thread, user, content):
          
          msg = cls.objects.create(thread=thread, sender=user, content=content)
          thread.userthread_set.exclude(user=user).update(deleted=False, unread=True)
          thread.userthread_set.filter(user=user).update(deleted=False, unread=False)
          message_sent.send(sender=cls, message=msg, thread=thread, reply=True)
          return msg
docs: Create a new reply for an existing Thread.
  
          Mark thread as unread for all other participants, and
          mark thread as read by replier.