I think you can acheive the "back reference" by defining an instance
method that returns a query object defined to search the other model
for references to itself. In fact I think that is all
ReferenceProperty does anyway.
Try something like this:
class ModelA(db.Model):
def modelB_list(self):
if self.key():
refs = ModelB.all()
refs.filter('a_id_ref =', self.key().id())
return refs
return None
class ModelB(db.Model):
a_ref_id = db.IntegerProperty()
ma = ModelA()
ma.put()
mb1 = ModelB(a_id_ref=ma.key())
mb2 = ModelB(a_id_ref=ma.key())
db.put([mb1,mb2])
for mb in ma.modelB_list:
print str(mb.key())
Robert
On Nov 29, 2009, at 5:17, 风笑雪 <[email protected]> wrote:
> Of course you can use id/name to instead of reference property, but
> you can get some benefit when using reference property.
>
> One is back reference. If a reference to b, b also reference to a, you
> need store a's key in b and b's key in a.
> But using reference property, you can automatically get a back
> reference.
> Once you need change the relationship, link a and c, you don't need
> change a, b, c at the same time(maybe a transaction), just change a's
> reference property to point to c, and everything has been done.
>
> 2009/11/29 MajorProgamming <[email protected]>:
>> I was wondering: isn't using reference properties a waste of space?
>> Wouldn't it make more sense to store the id (assuming not using
>> key_name) of the entity. After all, if the Kind is known, one can
>> easily generate the full key based on that. And with a reference
>> property it actually stores the _entire_ key (very long!).
>>
>> Why?
>>
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