[FIXED] Django Manager Chaining

Issue

I was wondering if it was possible (and, if so, how) to chain together multiple managers to produce a query set that is affected by both of the individual managers. I'll explain the specific example that I'm working on:

I have multiple abstract model classes that I use to provide small, specific functionality to other models. Two of these models are a DeleteMixin and a GlobalMixin.

The DeleteMixin is defined as such:

class DeleteMixin(models.Model):
    deleted = models.BooleanField(default=False)
    objects = DeleteManager()

    class Meta:
        abstract = True

    def delete(self):
        self.deleted = True
        self.save()

Basically it provides a pseudo-delete (the deleted flag) instead of actually deleting the object.

The GlobalMixin is defined as such:

class GlobalMixin(models.Model):
    is_global = models.BooleanField(default=True)

    objects = GlobalManager()

    class Meta:
        abstract = True

It allows any object to be defined as either a global object or a private object (such as a public/private blog post).

Both of these have their own managers that affect the queryset that is returned. My DeleteManager filters the queryset to only return results that have the deleted flag set to False, while the GlobalManager filters the queryset to only return results that are marked as global. Here is the declaration for both:

class DeleteManager(models.Manager):
    def get_query_set(self):
        return super(DeleteManager, self).get_query_set().filter(deleted=False)

class GlobalManager(models.Manager):
    def globals(self):
        return self.get_query_set().filter(is_global=1)

The desired functionality would be to have a model extend both of these abstract models and grant the ability to only return the results that are both non-deleted and global. I ran a test case on a model with 4 instances: one was global and non-deleted, one was global and deleted, one was non-global and non-deleted, and one was non-global and deleted. If I try to get result sets as such: SomeModel.objects.all(), I get instance 1 and 3 (the two non-deleted ones - great!). If I try SomeModel.objects.globals(), I get an error that DeleteManager doesn't have a globals (this is assuming my model declaration is as such: SomeModel(DeleteMixin, GlobalMixin). If I reverse the order, I don't get the error, but it doesn't filter out the deleted ones). If I change GlobalMixin to attach GlobalManager to globals instead of objects (so the new command would be SomeModel.globals.globals()), I get instances 1 and 2 (the two globals), while my intended result would be to only get instance 1 (the global, non-deleted one).

I wasn't sure if anyone had run into any situation similar to this and had come to a result. Either a way to make it work in my current thinking or a re-work that provides the functionality I'm after would be very much appreciated. I know this post has been a little long-winded. If any more explanation is needed, I would be glad to provide it.

Edit:

I have posted the eventual solution I used to this specific problem below. It is based on the link to Simon's custom QuerySetManager.

Solution

See this snippet on Djangosnippets: http://djangosnippets.org/snippets/734/

Instead of putting your custom methods in a manager, you subclass the queryset itself. It's very easy and works perfectly. The only issue I've had is with model inheritance, you always have to define the manager in model subclasses (just: "objects = QuerySetManager()" in the subclass), even though they will inherit the queryset. This will make more sense once you are using QuerySetManager.

Answered By - Scott