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 distance(self, pt):
        self.pt = pt
        x = self.x - self.pt.x
        y = self.y - self.pt.y
        return math.sqrt(x * x + y * y)