Solve the Downcasting-Problem of Django Models

In Django, I cannot downcast (attributes are missing). Any trivial solution breaks the Open-Closed Principle. This Solution not:

from django.db import models

class ParentModel(models.Model):
    type = models.CharField(max_length=255, default='item')
    class Meta:
        abstract = True
    def save(self, *args, **kwargs):
        self.type = self.type_str
        super().save(*args, **kwargs)
    @staticmethod
    def register_subtype(clazz):
        the_type = clazz.type_str
        if not hasattr(ParentModel, 'subclasses'):
            ParentModel.subclasses = {}
        ParentModel.subclasses[the_type] = clazz
    def downcast(self):
        if self.type not in ParentModel.subclasses:
            return self
        return ParentModel.subclasses[self.type].objects.get(pk=self.pk)

With the example Item:

class Item(ParentModel):
    """Stuff you can purchase"""
    type_str = 'item'
    name = models.CharField(max_length=255)
    ...

and it's subclass:

class Recipe(Item):
    type_str = 'recipe'
    duration = models.IntegerField(null=True)
    ...

I register the subclass, which is the key point to conform with the OCP:

from django.conf import settings
from django.apps import AppConfig

class RecipesConfig(AppConfig):
    name = 'recipes'
    def ready(self):
        from .models import Recipe
        from core.models import Item
        Item.register_subtype(Recipe)

Now, I can downcast an item, if it is a subclass:

def test_downcasting_with_subtype(self):
    Item.register_subtype(Recipe)
    Recipe.objects.create(name="recipe_name", duration=20)
    item = Item.objects.all()[0].downcast()
    self.assertIsInstance(item, Recipe)
    self.assertEqual(20, item.duration)  # subtype specific data loaded