This destroys data :-(

gistfile1.py

class Model(object):
    ...

    def save(self, ops=None):
        ops = dict(ops or ())
        for column in self.modified_columns:
            ops.setdefault('$set', {})[column] = self.data[column]
        rv = self.collection.find_and_modify({'_id': self.id}, ops)
        self.data.update(rv)