Django Group.objects monkey patch problem - 'NoneType' object has no attribute '_meta'

Question

I'm junior dev and I'm trying to monkey patch django.contrib.auth.models Group manager. (Python 2.7.15, Django 1.8)

There is my code:

class DefaultGroupManager(models.Manager):
    def get_queryset(self):
        tests = Test.objects.values()
        tests_ids = [test['virtual_group_id'] for test in tests]
        return super(DefaultGroupManager, self).get_queryset().exclude(id__in=tests_ids)


Group.objects = DefaultGroupManager()

Then I open python shell to test it:

python manage.py shell

from django.contrib.auth.models import Group

t = Group.objects.all()

Right after this command i'm getting error:

  File "<console>", line 1, in <module>
  File "/home/adrian/Dokumenty/Pycharm Projects/backend/venv/local/lib/python2.7/site-packages/django/db/models/manager.py", line 228, in all
    return self.get_queryset()
  File "/home/adrian/Dokumenty/Pycharm Projects/backend/registration/models.py", line 13, in get_queryset
    return super(DefaultGroupManager, self).get_queryset().exclude(id__in=tests_ids)
  File "/home/adrian/Dokumenty/Pycharm Projects/backend/venv/local/lib/python2.7/site-packages/django/db/models/query.py", line 686, in exclude
    return self._filter_or_exclude(True, *args, **kwargs)
  File "/home/adrian/Dokumenty/Pycharm Projects/backend/venv/local/lib/python2.7/site-packages/django/db/models/query.py", line 695, in _filter_or_exclude
    clone.query.add_q(~Q(*args, **kwargs))
  File "/home/adrian/Dokumenty/Pycharm Projects/backend/venv/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1310, in add_q
    clause, require_inner = self._add_q(where_part, self.used_aliases)
  File "/home/adrian/Dokumenty/Pycharm Projects/backend/venv/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1338, in _add_q
    allow_joins=allow_joins, split_subq=split_subq,
  File "/home/adrian/Dokumenty/Pycharm Projects/backend/venv/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1150, in build_filter
    lookups, parts, reffed_expression = self.solve_lookup_type(arg)
  File "/home/adrian/Dokumenty/Pycharm Projects/backend/venv/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1036, in solve_lookup_type
    _, field, _, lookup_parts = self.names_to_path(lookup_splitted, self.get_meta())
  File "/home/adrian/Dokumenty/Pycharm Projects/backend/venv/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 246, in get_meta
    return self.model._meta
AttributeError: 'NoneType' object has no attribute '_meta'

I have no idea whats wrong :(

I will appreciate any help!

Answer 1

You should not monkey patch like that: Django has some logic in place at the construction of the class, that will "inject" extra data in the fields, managers, etc.

By simply monkey patching, you omit those steps. A manager has however a contribute_to_class method, so you can monkey patch it like:

dgm = DefaultGroupManager()
dgm.contribute_to_class(Group, 'objects')
Group.objects = dgm

That being said, in my opinion it typically results in a lot of troubly if you overwrite the .objects manager, since a lot of Django code will assume that with Group.objects, you get all objects. Furthermore it is still possible that a lot of Django tooling will fetch all Groups, since these might work with the Model._base_manager [Django-doc].

So although it is (probably) not violating any contracts, it will probably result in more trouble than it is worth.

If you run a test, typically you do that in an isolated environment. Furthermore integration tests, typically run on a separate database as well.