mydb.py

from google.appengine.ext import db

class ReferenceProperty(db.ReferenceProperty):
    """A property that represents a many-to-one reference to another model.

    This class has serveral enhancement on
    google.appengine.ext.db.ReferenceProperty.

    1. Its object returns None when referenced entity does not exist, nstead
    of raising ReferencePropertyResolveError.

    2. It accepts access_by_key keyward argument on instantiation. If it is
    True, you can access key of referenced object by <propertyname>_key
    attribute.
    """
    def __init__(self,
                 reference_class=None,
                 verbose_name=None,
                 collection_name=None,
                 access_by_key=False,
                 **attrs):
        # set this value to refer from __property_config__ method.
        self._access_by_key = access_by_key
        super(ReferenceProperty, self).__init__(verbose_name, **attrs)

    def __property_config__(self, model_class, property_name):
        """creates _key suffixed property if self._access_by_key is True"""
        super(ReferenceProperty, self).__property_config__(model_class,
                                                           property_name)
        if self._access_by_key:
            def get_key(self):
                property = getattr(model_class, property_name)
                return property.get_value_for_datastore(self)
            setattr(model_class, "%s_key" %property_name,
                    property(get_key))

    def __get__(self, *args, **kw):
        try:
            return super(ReferenceProperty, self).__get__(*args, **kw)
        except db.ReferencePropertyResolveError:
            return None