update() with manipulate=True inserts _id in update document

when manipulate is set False, calling collection.update(spec, document, manipulate=False) is ok. But when manipulate is set to True, calling collection.update(spec, document, manipulate=True) raise Exception.

here is the test code.

# -*- coding: utf-8 -*-

import pymongo

class MySONManipulator(pymongo.son_manipulator.SONManipulator):

    def transform_incoming(self, son, collection):

        print son

        return son

def test_pymongo():

    client = pymongo.MongoClient()

    db = client['test']

    # db.add_son_manipulator(MySONManipulator())

    collection = db['test']

    collection.remove()

    collection.insert({'name': 'a'})

    collection.update({'name': 'a'}, {'word': 'hello world'}, manipulate=True)

test_pymongo()

it raise: pymongo.errors.OperationFailure: cannot change _id of a document old:

new:

.

And i found one solution is to delete field '_id' of document in method update().

if manipulate:
document = self.__database._fix_incoming(document, self)

1. delete '_id' here
   if '_id' in document:
   del document['_id']