Automatically downcast to subclass using django-model-utils

问题

I have multiple user models .. all inheriting a Base model with a custom manager

models.py

class BaseUser(models.Model):
    [...]

    objects = UserManager()


class StaffUser(BaseUser):
    [...]

class Customer(BaseUser):
    [...]

managers.py

from model_utils.managers import InheritanceManager

class UserManager(..., InheritanceManager):
    [...]

By inheriting the InheritanceManager from django-model-utils, I can do automatic downcasting for inherited models. So for example, if I have 3 objects, one of each user type:

user = BaseUser.objects.select_subclasses()

[Customer: customer@example.com, BaseUser: studio@example.com, StaffUser: staff@example.com]

To do that, I had to explicitly call .select_subclasses(),

But I want to do the downcasting automatically without having to call .select_subclasses()

So I tried:

class UserManager(..., InheritanceManager):

    def get_queryset(self, *args, **kwargs):
        queryset = super(UserManager, self).get_queryset()
        return queryset.get_subclass(*args, **kwargs)

But now when I try to use:

user = EmailUser.objects.all()

I get this:

MultipleObjectsReturned: get() returned more than one BaseUser -- it returned 3!

Is it possible to do automatic downcasting for inherited models using django-model-utils?

回答1:

I figured it out!

managers.py

from model_utils.managers import InheritanceQuerySet

class UserManager([...]):

    def get_queryset(self):
        return InheritanceQuerySet(self.model).select_subclasses()

I didn't have to inherit InheritanceManager, instead use the InheritanceQuerySet.

来源：https://stackoverflow.com/questions/25098471/automatically-downcast-to-subclass-using-django-model-utils