Using cls.__dict__ sometimes returns <class 'django.db.models.manager.ManagerDescriptor'>

[benjaoming]

I'm having problems that I didn't get to the bottom of, except that reverting back to getattr instead of accessing via __dict__ is the remedy.

Problem was that a custom manager was not set, and we were defaulting back to TreeManager.

The culprit was this:

                        # HACK: avoid using getattr(cls, attr)
                        # because it calls __get__ on descriptors, which can cause nasty side effects
                        # with more inconsiderate apps.
                        # (e.g. django-tagging's TagField is a descriptor which can do db queries on getattr)
                        # ( ref: http://stackoverflow.com/questions/27790344 )
                        obj = cls.__dict__[attr]

It was causing django-wiki/django-wiki#404

PR coming in......

[benjaoming]

Here's the exact cause: a01b157

But I realize it fixes another important issue.

[craigds]

Hi Ben

The last comment on the django-wiki ticket implies this is something to do with .none() querysets, but I'm not quite sure how that relates to ManagerDescriptor.

Perhaps you could you clarify a bit?

[benjaoming]

@craigds thanks for getting back

I had to halt the investigation, but I'm trying to figure it out. I was building a test case for django-mptt but it somehow succeeds (oh no!!! type(Model.objects.all()[0].get_children()) is CustomQuerySet) with the current codebase, but the exact same thing in django-wiki is failing with the same structure (type(Model.objects.all()[0].get_children()) is TreeQuerySet instead of CustomQuerySet).

Human rant: So the problem is caused by django-wiki's custom manager... it something isn't playing the right way, since __dict__[attr] is returning a ManagerDescriptor (that's a class that will just raise an exception about not using manager functions directly on an object), and then django-mptt defaults to TreeManager because it doesn't find the custom manager inheriting from TreeManager.

[benjaoming]

Btw thanks for a great project again!!!

[benjaoming]

Okay I've found the problem, it's essentially this:

class MyModel(MPTTModel):
    objects = MyCustomManager()
    _default_manager = objects  # <- THAT IS THE PROBLEM

[benjaoming]

...now I just need to figure out why that line was introduced

[benjaoming]

Confirming that django-wiki is working after commenting out the _default_manager = objects line.

[benjaoming]

Also confirming that I can make the django-mptt tests break by introducing this line... wondering if it's relevant since googling stuff about this issue reveals issues about older django versions.

[benjaoming]

Okay, it's all in #370

I would add that it seems removing _default_manager in the model in django-wiki isn't harmful, I'll try it out and report back.

[benjaoming]

Argh, dammit, I wanted Travis to be showing off how the tests were failing in 2b47ffd and then how they are now working in 92737e9