Why does query __isnull=True cause DatabaseError? - google-app-engine

Question

When I do a filter on a ForeignKey field with __isnull=True, this exception is raised:
DatabaseError: This query is not supported by the database.
However, __isnull=False on ForeignKey works as long as there are no other inequality filters (which I would expect). And __isnull=True works for other field types.
So why does __isnull=True not work on ForeignKey? It seems that DBIndexer tries to make it work as shown here:
https://github.com/django-nonrel/django-dbindexer/blob/dbindexer-1.4/dbindexer/backends.py
But then there is an exception in djangotoolbox:
File "/Users//Documents/workspace/-gae-dev/src/django/db/models/query.py", line 107, in _result_iter
self._fill_cache()
File "/Users//Documents/workspace/-gae-dev/src/django/db/models/query.py", line 774, in _fill_cache
self._result_cache.append(self._iter.next())
File "/Users//Documents/workspace/-gae-dev/src/django/db/models/query.py", line 275, in iterator
for row in compiler.results_iter():
File "/Users//Documents/workspace/-gae-dev/src/djangotoolbox/db/basecompiler.py", line 337, in results_iter
results = self.build_query(fields).fetch(
File "/Users//Documents/workspace/-gae-dev/src/djangotoolbox/db/basecompiler.py", line 428, in build_query
self.check_query()
File "/Users//Documents/workspace/-gae-dev/src/djangotoolbox/db/basecompiler.py", line 409, in check_query
raise DatabaseError("This query is not supported by the database.")
I did come across the following commented-out test case in djangoappengine, and am wondering if is referring to the same issue?
def test_is_null(self):
self.assertEquals(FieldsWithOptionsModel.objects.filter(
floating_point__isnull=True).count(), 0)
FieldsWithOptionsModel(
integer=5.4, email='shinra.tensai#sixpaths.com',
time=datetime.datetime.now().time()).save()
self.assertEquals(FieldsWithOptionsModel.objects.filter(
floating_point__isnull=True).count(), 1)
# XXX: These filters will not work because of a Django bug.
# self.assertEquals(FieldsWithOptionsModel.objects.filter(
# foreign_key=None).count(), 1)
# (it uses left outer joins if checked against isnull)
# self.assertEquals(FieldsWithOptionsModel.objects.filter(
# foreign_key__isnull=True).count(), 1)

Answer 1

Alex Burgel on the NonRel project set me straight:
The NonRel/dbindexer project fixes this query (which otherwise doesn't work due to this Django bug: https://code.djangoproject.com/ticket/10790). To set up dbindexer:
of course, add it to INSTALLED_APPS
also in settings.py, set DATABASES['default]['ENGINE']= 'dbindexer'
also in settings.py, set DBINDEXER_BACKENDS to use FKNullFix. For example:
DBINDEXER_BACKENDS = (
'dbindexer.backends.BaseResolver',
'dbindexer.backends.FKNullFix',
'dbindexer.backends.InMemoryJOINResolver',
'dbindexer.backends.ConstantFieldJOINResolver',
)