Hook for default QuerySet filtering defined on the QuerySet itself.

[Loic Bistuer]

Django 1.7 brought managers automatically created from QuerySet which replaces defining a custom manager for the purpose of defining reusable methods. Refs #20625.

One use-case remains inelegant: using a custom QuerySet with default QuerySet customization/filtering:

BaseCustomManager = Manager.from_queryset(CustomQueryset)

class CustomManager (BaseCustomManager ):
    def get_queryset(self):
        queryset = super(Manager, self).get_queryset()
        return queryset.filter(...)

This ticket proposes adding a hook on QuerySet to enable this without requiring a custom Manager.

[Loic Bistuer]

It's worth noting that QuerySet.__init__() can't be used for providing such initialization/customization:

• QuerySet are often cloned, and the customization should only ever apply once.
• QuerySet methods return a cloned QuerySet instance and __init__ can't return a different instance.

So far the best option I can think of is a hook called externally by the manager.

POC with a QuerySet.get_initial_queryset() method ​https://github.com/loic/django/compare/ticket23533

[Simon Charette]

It'd be good to explore other implementations than that get_initial_queryset hook as that bi-directionnaly couples Querysets with managers (even more than the existing as_manager method) and prevents reuse of the same queryset class for different managers of the same model.

Approaches such as CustomQueryset.as_manager(filter=Q(is_active=True)) and CustomQueryset.filter(is_active=True).as_manager() (this would require marking some methods class_or_instance_method at __init_subclass__ time to capture the calls and apply them at Manager.contribute_to_class / app readyness time) seem more valuable as they don't require overriding any methods.

In other words, the get_initial_queryset hook saves you from defining a manager but you still have to define a method. It also ties your queryset class to a single manager usage with seems wrong? What if you want to use your custom queryset class with two different filters sets

class FooQueryset(models.QuerySet):
    def is_bar(self):
        return self.filter(bar=True)

class FooBazQueryset(FooQueryset):
    def get_initial_queryset(self):
        return self.filter(baz=True)

class FooBatQueryset(FooQueryset):
    def get_initial_queryset(self):
        return self.filter(bat=True)

class Foo(models.Model):
    bar = models.BooleanField()
    baz = models.BooleanField()
    bat = models.BooleanField()

    objects = FooQueryset.as_manager()
    baz_objects = FooBazQueryset.as_manager()
    bat_objects = FooBatQueryset.as_manager()

Compare that with

class FooQueryset(models.QuerySet):
    def is_bar(self):
        return self.filter(bar=True)

class Foo(models.Model):
    bar = models.BooleanField()
    baz = models.BooleanField()
    bat = models.BooleanField()

    objects = FooQueryset.as_manager()
    baz_objects = FooQueryset.filter(baz=True).as_manager()
    bat_objects = FooQueryset.filter(bat=True).as_manager()