def get_last_tid(self):
        try:
            spec = {'tid': {'$gt': 0}}
            sort = [('tid', -1)]
            row = self.db.docs.find_one(spec, sort=sort)
        except Exception as e:
            raise DatabaseException(e)
        if row:
            return row['tid']
        return 0
def serialize(self, value, display=False):
        lookup = {v: k for k, v in self.levels.items()}
        if value in lookup:
            return encode(lookup[value])
        return ""