AppEngine: Query datastore for records with value

Question

I created a new property for my db model in the Google App Engine Datastore.

Old:

class Logo(db.Model):
  name = db.StringProperty()
  image = db.Blo         


        

Answer 1

According to the App Engine documentation on Queries and Indexes, there is a distinction between entities that have no value for a property, and those that have a null value for it; and "Entities Without a Filtered Property Are Never Returned by a Query." So it is not possible to write a query for these old records.

A useful article is Updating Your Model's Schema, which says that the only currently-supported way to find entities missing some property is to examine all of them. The article has example code showing how to cycle through a large set of entities and update them.

Answer 2

Maybe this has changed, but I am able to filter records based on null fields.

When I try the GQL query SELECT * FROM Contact WHERE demo=NULL, it returns only records for which the demo field is missing.

According to the doc http://code.google.com/appengine/docs/python/datastore/gqlreference.html:

The right-hand side of a comparison can be one of the following (as appropriate for the property's data type): [...] a Boolean literal, as TRUE or FALSE; the NULL literal, which represents the null value (None in Python).

I'm not sure that "null" is the same as "missing" though : in my case, these fields already existed in my model but were not populated on creation. Maybe Federico you could let us know if the NULL query works in your specific case?