You can construct a __key__ only query and count them up.
Then you don't have to construct all the A objects just for counting.
result = A.all(keys_only=True).filter('refprop =', b.key()).fetch(1000)
numA = len(result)
2009/6/22 Nick Johnson (Google) <nick.john...@google.com>:
> Hi johntray,
>
> On Sun, Jun 21, 2009 at 5:47 PM, johntray <john.tur...@gmail.com> wrote:
>>
>> Well yes I could add a count property to the B objects, but I'm really
>> trying to understand more about how ReferenceProperty works.
>
> ReferenceProperty's collection attributes are just syntactic sugar for
> creating and executing a query yourself. As such, all the same limitations
> apply. In the case where you call len() on it, I believe this will result in
> a count query being executed, which doesn't require Python to decode all the
> entities, but does require the datastore to fetch all the index rows.
> [snip]
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