code: def update(postid, userid, rating):
          '''
          Update the rating of certain post and user.
          The record will be created if no record exists.
          '''
          rating_recs = TabRating.select().where(
              (TabRating.post_id == postid) & (TabRating.user_id == userid)
          )
          if rating_recs.count() > 0:
              MRating.__update_rating(rating_recs.get().uid, rating)
          else:
              MRating.__insert_data(postid, userid, ratingf update(postid, userid, rating):
          '''
          Update the rating of certain post and user.
          The record will be created if no record exists.
          '''
          rating_recs = TabRating.select().where(
              (TabRating.post_id == postid) & (TabRating.user_id == userid)
          )
          if rating_recs.count() > 0:
              MRating.__update_rating(rating_recs.get().uid, rating)
          else:
              MRating.__insert_data(postid, userid, rating)
docs: Update the rating of certain post and user.
          The record will be created if no record exists.