How to extend a normal model and make it polymorphic?

[jorgeas80]

Here, my use case:

from django.db import models
from polymorphic.models import PolymorphicModel

class BaseClassWithLotsOfStuff(models.Model):
    # Here, a lot of existing atributes (variables and methods)
    # ...

class InheritedClassOne(BaseClassWithLotsOfStuff):
    # A lot of stuff here too...
    #...

Those classes are part of a legacy project, and I can't change them to PolymorphicModel (too many possible side effects). But now, I need to extend the project, by inheriting more classes from the base class. And I want to use django-polymorphic.

So, my point: why don't I create a new base class, inherit that class from the original BaseClassWithLotsOfStuff (to allow using the goodies it contains), make it polymorphic and inherit the rest from it?

class NewBaseClass(BaseClassWithLotsOfStuff, PolymorphicModel):
    # New stuff here
    # ...


class InheritedClassTwo(NewBaseClass):
    # We are polymorphic now!
    # ...

So I tried that. And when running makemigrations, I got this error

AssertionError: PolymorphicModel: "InheritedClassTwo._default_manager" manager is of type "Manager", but must be a subclass of PolymorphicManager

I guess the problem is PolymorphicModel defines a custom manager, amongs other things, and it crashes because of the double inheritance in BaseNewClass.

So, does my point make sense? Can it be done in a different way?

[vdboor]

This will work but the PolymorphicModel must come before the base class in the inheritance MRO:

class NewBaseClass(PolymorphicModel, BaseClassWithLotsOfStuff):
    # New stuff here
    # ...