def update_count(cat_id):
        '''
        Update the count of certain category.
        '''
        # Todo: the record not valid should not be counted.
        entry2 = TabTag.update(
            count=TabPost2Tag.select().where(
                TabPost2Tag.tag_id == cat_id
            ).count()
        ).where(TabTag.uid == cat_id)
        entry2.execute()
Update the count of certain category.