def get_next_id(self):
        try:
            query = {'_id': 'last_id'}
            update = {'$inc': {'id': 1}}
            fn = self.db.internal.find_and_modify
            row = fn(query, update, upsert=True, new=True)
        except Exception as e:
            raise DatabaseException(e)
        return row['id']
def commit(self):
        context = self.pop()
        if self:
            # savepoint
            self[-1]['cbs'].extend(context['cbs'])
            self[-1]['dirty'] = self[-1]['dirty'] or context['dirty']
        else:
            # transaction
            for func, args, kwargs in context['cbs']:
                func(*args, **kwargs)