Appengine reverse reference problem

5
votes

According to the docs: http://code.google.com/appengine/docs/python/datastore/datamodeling.html#References the automatically created reverse reference object is a Query object, so there is possible iteration over it and making fetch calls.

But: I have one model:

class User(db.Model):
   name = db.StringProperty()
   ...

and second model:

class Thing(db.Model):
    owner = db.ReferenceProperty(User)
    ...

And when I try to access reverse reference:

for thing in user.thing_set:
    ...

or:

user.thing_set.fetch(100)

I get an exception like this:

<type 'exceptions.TypeError'>: '_ReverseReferenceProperty' object is not iterable

or like this:

<type 'exceptions.AttributeError'>: '_ReverseReferenceProperty' object has no attribute 'fetch'

Am I doing something wrong or there was some change in appengine? I'm pretty sure that previously it worked like a Query. There is even an example on the docs page, that shows the same usage as mine:

for obj in obj1.secondmodel_set:
    # ...

Additionaly getting the query without reverse reference works ok:

things = Thing.all().filter('owner =', user)
1
pythongoogle-app-enginegoogle-cloud-datastore
Can you identify any particular circumstance that give rise to the to first exception an which give rise to the second exception? - Adam Crossland

1 Answers

1
votes

Both methods (iterating and fetch) should work. To debug, you might want to log (or print):

print dir(user)
[..., 'thing_set', ...]

print dir(user.thing_set)
[..., '__iter__', ... , 'fetch', ...]

just to see what the objects contain... and that might give you a hint of what could be going wrong.

A couple of ideas: