gistfile1.py

    def save(self, message):
        """Append new comment to the message, increment comment_count."""
        user = current_user._get_current_object()

        # Set the comment's pk to -1 so we can find it in the comment list
        # (We can't guarantee it will be last when we go to access it)
        comment = Comment(body=clean_html_input(self.body.data),
                          author=user, pk=-1)

        message_qs = Message.objects(id=message.id)
        message_qs.update(push__comments=comment)
        message_qs.update(inc__cache__comments_count=1)
        message_qs.update(set__updated_at=datetime.datetime.now())

        comments_length = len(message.comments)
        if comments_length:
            # Go backwards until we find the first comment with a positive pk
            # (because the most recent comment might still have a pk of -1,
            #  after just being pushed to msg.comments.)
            prev_comment_index = comments_length - 1
            for i in reversed(xrange(comments_length)):
                if message.comments[i].pk:
                    prev_comment_pk = message.comments[i].pk
                    break
        else:
            prev_comment_index = 0
            prev_comment_pk = 1

        updated_message = message_qs.first()
        no_pk_comments = []
        for new_comment_index in xrange(len(updated_message.comments)):
            if updated_message.comments[new_comment_index].pk == -1:
                no_pk_comments.append(new_comment_index)

        for no_pk_comment_index in no_pk_comments:
            distance = no_pk_comment_index - prev_comment_index
            new_comment_pk = prev_comment_pk + distance

            kwargs = {"set__comments__%d__pk" % no_pk_comment_index: new_comment_pk}
            message_qs.update(**kwargs)

        # Add this message to the user following list.
        updated_message.add_to_following_list(user)

        # Increment user comment_count
        user.cache.comment_count += 1
        user.save()

        return new_comment_pk