That's too much typing so I defined the following.
# 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 <[EMAIL PROTECTED]> wrote:
> Mazia is correct, there is no guarantee that this will work. In
> fact, it is very likely it will not work in the future. We may in the
> future implement lazy loading so that you can access the key directly
> without loading the entity (a.C.key()) however, for now, this is
> guaranteed to work:
>
> A.C.get_value_for_datastore(a)
>
> On Sep 29, 6:23 pm, Andy Freeman <[EMAIL PROTECTED]> wrote:
>
>
>
> > Is there an approved way to copy a ReferenceProperty from one entity
> > to another without fetching the referred to entity from the datastore?
>
> > 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.- Hide quoted text -
>
> - Show quoted text -
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