Question

Using the python version of GAE and extending models from db.Model, how do you fetch entities where an attribute equals None or does not exist?

#This works
#Fetch 10 entities where duration == 0.0
entities = MyModel.all().filter('duration = ', 0.0).fetch(10)

#This doesn't. How can I do the equivalent?
#Fetch 10 entities where duration == None
entities = MyModel.all().filter('duration = ', None).fetch(10)

Answer 1

You have entities without duration property (can’t be filtered because index can’t refer to them) and entities with duration set to None (can be filtered).

Since you have changed MyModel schema, you should fix the entities stored without duration property with something like this:

entities = MyModel.all()
for entity in entities :
  if not entity.duration :
    entity.duration = None
    entity.put()

Have a look to appengine-mapreduce library to accomplish this long running task.