How to identify objects with broken reference properties in google app engine

Question

How do you correctly check for broken reference properties in google app engine?

Example:

class User (db.Model):
    userName = db.StringProperty(multiline=False)
class Foo (db.Model):
    user = db.ReferenceProperty(User, collection_name="user_foo")

1. A User object was created.
2. A Foo object was created.
3. The corresponding reference property in User was then deleted.

Answer 1

As suggested by Daniel Roseman in the comments:

"iterate through all Foos and access item.user, and [check] if that ResolveError is raised"

from google.appengine.api import datastore_errors

all_foo = Foo.all()
for bar in all_foo:
    try:
        user_refProperty = bar.user
    except datastore_errors.Error, e:
        if e.args[0][0:40] == "ReferenceProperty failed to be resolved:":
            bar.delete()
            self.response.out.write('deleted due to bad reference property')
        else: raise