def get_rating(postid, userid):
        '''
        Get the rating of certain post and user.
        '''
        try:
            recs = TabRating.select().where(
                (TabRating.post_id == postid) & (TabRating.user_id == userid)
            )
        except:
            return False
        if recs.count() > 0:
            return recs.get().rating
        else:
            return False
Get the rating of certain post and user.