My attempt to do idempotent Mongo writes with MongoKit

document.py

# -*- coding: utf-8 -*-
# Copyright (c) 2012, Scott Reynolds
# All rights reserved.

from mongokit import Document, UpdateQueryError
from pymongo.errors import OperationFailure
from bson import BSON
import hashlib

class IdempotentDocument(Document):
    """
    Document that is able to ensure that any writes are idempotent and when the
    document write is not idempotent, throws an Exception.
    """
    structure = {
        '_versioned_id': unicode,
        }

    def save(self, validate=None, safe=True, *args, **kwargs):
        """
        save the document into the db.
        """
        old_version = ''
        if '_versioned_id' in self and self['_versioned_id']:
            old_version = self['_versioned_id']

        # Find the properties of the document that will be used for the hash of
        # the document and the properties to save to the document.
        hashable_properties = {}
        updates = {}
        for key, value in self.iteritems():
            # don't update the _versioned_id. it will be set later on below with
            # the new _versioned_id. This pattern allows this property to be
            # altered yet! be saved properly as the sha1 of the document.
            if key != '_id' and key != '_versioned_id':
                updates[key] = value
                hashable_properties[key] = value

        # Version id is the sha1 of the hashable contents.
        # @TODO: would be nice to make hashable properties be definable on the
        # model.
        new_version = unicode(hashlib.sha1(BSON.encode(hashable_properties)).hexdigest())
        self['_versioned_id'] = updates['_versioned_id'] = new_version

        if validate is True or (validate is None and self.skip_validation is False):
            self.validate(auto_migrate=False)
        else:
            if self.use_autorefs:
                self._make_reference(self, self.structure)

        # Basically, if this is an update AND the old one doesn't have the same
        # sha1 as the new one.
        if old_version and old_version != new_version:
            self['_versioned_id'] = new_version
            self._process_custom_type('bson', self, self.structure)
            new_data = self.collection.find_and_modify({'_versioned_id': old_version},
                                 {'$set': updates}, new=True)
            self._process_custom_type('python', self, self.structure)
            if not new_data:
                raise UpdateQueryError("Document has changed since it was last loaded")
        # This means it is a new document not loaded from the database. Try to
        # save it and if it fails, that means someone beat us too it.
        elif not old_version:
            self['_id'] = new_version
            try:
                self.collection.find_and_modify({'_id': self['_id'],
                                                 '_versioned_id': new_version},
                                                {'$set': updates},
                                                new=True, upsert=True)
            except OperationFailure:
                raise UpdateQueryError("Document has already been created")