App Engine: ReferencePropertyResolveError in Google App Engine?

Question

Please help me to understand Why this error on App Engine (Python 2.5, High Replication) ?

Traceback (most recent call last):
File "/base/python_runtime/python_lib/versions/1/google/appengine/ext/webapp/_webapp25.py", line 703, in __call__ 
    handler.post(*groups)
File "/base/data/home/apps/s~apptest/1.354937136776013205/main.py", line 963, in post
    get_purchases_for_user(pur.user, pur.car.model)
File "/base/python_runtime/python_lib/versions/1/google/appengine/ext/db/__init__.py", line 3592, in __get__
    reference_id.to_path())
ReferencePropertyResolveError: ReferenceProperty failed to be resolved: [u'User', 374836L]

Context: The application is about recommendations to buy cars.

When this happens: Around 2 times per each 100 transactions approximately, so really I don't know when nor why.

Where the object pur in the code is defined as:

allPurchases = models.Purchase.all()
for pur in allPurchases:
   car = get_purchases_for_user(pur.user, pur.car.model)

Also I put this code before and I got the same error, of course referencing to this line

logging.error("Object user: %s, Car Model %s" % {str(pur.user), pur.car.model})

So, the problem is not in the get_purchases_for_user function

And these are the models in the datastore:

class User(db.Model):
    username = db.StringProperty(
        required=True)
    name = db.StringProperty(
        required=True)
    email = db.EmailProperty(
        required=True)
    password = db.StringProperty(
        required=True)

class Car(db.Model):
    model = db.StringProperty()
    name = db.StringProperty()
    year = db.DateTimeProperty()

class Purchase(db.Model):
    user = db.ReferenceProperty(User,
        collection_name='recommendations')
    car = db.ReferenceProperty(Car,
        collection_name='recommendations')
    when = db.DateTimeProperty(
        auto_now=True)

Did you confirm you actually have a User entity with the ID 374836? — Wooble
no, I haven't. But how the application can call to an entity that never has existed — coto
My guess would be that it existed at the time the ReferenceProperty was created and was then deleted without a thought for referential integrity. Unless you did something crazy like creating the Key in the ReferenceProperty programmatically without it pointing to an actual entity. — Wooble
impossible. The application never delete an user. I am completely sure about of that — coto
So, did you confirmed that user exists? It takes only 2 minutes and number of assumptions of possible things that can go wrong will be halved. — Ski

Nick Johnson Nick Johnson · Accepted Answer · 2011-12-09T19:04:08

Your entity has a ReferenceProperty to a User entity that doesn't exist - most likely because it has been deleted. You can add code to catch this particular exception and deal with it in whatever way is appropriate in your application.

App Engine: ReferencePropertyResolveError in Google App Engine?

2 Answers