问题
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.BlobProperty()
New:
class Logo(db.Model):
name = db.StringProperty()
image = db.BlobProperty()
is_approved = db.BooleanProperty(default=False)
How to query for the Logo records, which to not have the 'is_approved' value set? I tried
logos.filter("is_approved = ", None)
but it didn't work. In the Data Viewer the new field values are displayed as .
回答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.
回答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?
来源：https://stackoverflow.com/questions/598605/appengine-query-datastore-for-records-with-missing-value