Using Google App Engine for Python. It seems that when you had connected datastore entities under the old db model you could rather easily get a list of connected entities, but I can't seem to do the same thing with ndb.
username = ndb.StringProperty()
owner = ndb.KeyProperty(User)
name = ndb.StringProperty()
photos = ndb.StringProperty(repeated=True)
collections = user.collections
AttributeError: 'User' object has no attribute 'collections'
ndb does not create back-references. But it's just as easy to do the query forwards: