In other words,

class C(db.Model):
    ...

class A(db.Model):
    C = db.ReferenceProperty(reference_class=C)

class B(db.Model):
    C = db.ReferenceProperty(reference_class=C)

a = {some query}
b = B()
# I would like to do the next assignment without retrieving the
relevant entity
# from the data store.
b.C = a.C

Yes, I know that a._C.id() can be used to read the id, but that
doesn't seem to be part of the documented interface.

Hi,

The method that you are using currently works, but there is no guarantee
that it will continue to work in the future - by which I mean that the
current API design only specifies that ReferenceProperty is fetched from the
datastore when referenced.

We've had requests that there be a way to expose only the key of the
reference, and not the actual entity, and I think this is a valid feature
request.  I don't believe it's filed in the tracker, so you should do that,
and those interested in this feature should star the issue.

-Marzia

  A.C.get_value_for_datastore(a).key()

On Sep 29, 6:23 pm, Andy Freeman <ana...@earthlink.net> wrote:

  A.C.get_value_for_datastore(a)

  It is not necessary to call 'key()' because the object that gets
returns IS the key.

On Oct 2, 3:02 pm, Rafe <slobberch...@gmail.com> wrote:

# AddKeyReference is a __metaclass__ for db.Model subclasses that adds
# a property named x_key for every db.ReferenceProperty named x in the
# classes (and their subclasses).  x_key's value is x's db.Key, but is
# produced without fetching x's model instance from the datastore.
# x_key can also be set using None, a string, or a db.Key.
class AddKeyReference(db.Model.__metaclass__):
    def __init__(cls, namestring, bases, dct):
        super(AddKeyReference, cls).__init__(namestring, bases, dct)
        for (k,v,) in dct.iteritems():
            if isinstance(v, db.ReferenceProperty):
                key_name = k + '_key'
                if hasattr(cls, key_name):
                    raise db.DuplicatePropertyError(
                        '%s can not be redefined for ReferenceProperty
key' %
                        (key_name,))
                # save key_name, v's current value - all iterations
use the same variables
                def set(instance, value, key_name=key_name, r_p=v):
                    if isinstance(value, basestring):
                        value = db.Key(value)
                    if not ((None is value) or isinstance(value,
db.Key)):
                        raise db.BadKeyError('%s can not be %s' %
                                             (key_name, str(value),))
                    r_p.__set__(instance, value)
                setattr(cls, key_name,
                        property(lambda instance, r_p=v:

r_p.get_value_for_datastore(instance),
                                 set))

# example
class Test(db.Model):
    __metaclass__ = AddKeyReference
    xx = db.SelfReferenceProperty()

test = Test()
test.put()
assert None is test.xx
assert None is test.xx_key

test.xx = test
assert test.xx_key == test.key()

test.xx = None
assert None is test.xx_key

test.xx_key = test.key()
# at this point test.xx.xx will cause a datastore fetch

On Oct 2, 3:02 pm, Rafe <slobberch...@gmail.com> wrote: