Question

I have Exhibit objects which reference Gallery objects both of which are stored in the Google App Engine Datastore.

How do I order the Exhibit collection on each Gallery object when I get around to iterating over the values (ultimately in a Django template)?

i.e. this does not work

 class Gallery(db.Model):   title = db.StringProperty()   position = db.IntegerProperty()  class Exhibit(db.Model):   gallery = db.ReferenceProperty(Gallery, collection_name='exhibits')   title = db.StringProperty()   position = db.IntegerProperty()  galleries = db.GqlQuery('SELECT * FROM Gallery ORDER BY position') for gallery in galleries:   gallery.exhibits.order('position')  # ... send galleries off the the Django template  

When rendered in the template, the galleries are correctly ordered but the exhibits are not.

Answer 1

Instead of relying on the collection property App Engine creates, you need to construct your own query:

exhibits = Exhibit.all().filter(‘gallery =’, gallery).order(‘position’)

Or equivalently, in GQL:

exhibits = db.GqlQuery(‘SELECT * FROM Exhibit WHERE gallery = :1 ORDER BY position’, gallery)

If you want to be able to do this from inside the template, rather than passing in a list-of-lists of exhibits, you can define a simple method on the Gallery object that executes this query, and reference it from the template (Eg, {{gallery.exhibits_by_position}} will execute exhibits_by_position() on the Gallery object, which can then perform the query above).

If you’re concerned about the speed implications of this, don’t worry: The collection property App Engine creates is simply syntactic sugar for this.