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 _visit_migrate_unique_constraint(self, *p, **k):
    """Drop the given unique constraint

    The corresponding original method of sqlalchemy-migrate just
    raises NotImplemented error

    """

    self.recreate_table(p[0].table, omit_uniques=[p[0].name])