Simple Django Polymorphic

simple-django-polymorphic.py

# http://stackoverflow.com/questions/5360995/polymorphism-in-django-models
# http://stackoverflow.com/questions/929029/how-do-i-access-the-child-classes-of-an-object-in-django-without-knowing-the-nam/929982#929982

from django.db import models
from django.contrib.contenttypes.models import ContentType
from django.db.models.query import QuerySet


class SubclassingQuerySet(QuerySet):
    def __getitem__(self, k):
        result = super(SubclassingQuerySet, self).__getitem__(k)
        if isinstance(result, models.Model):
            return result.cast()
        return result

    def __iter__(self):
        for item in super(SubclassingQuerySet, self).__iter__():
            yield item.cast()


class PolymorphicManager(models.Manager):
    def get_queryset(self):
        return SubclassingQuerySet(self.model)

class PolymorphicModel(models.Model):
    """
    An abstract base class that provides a ``real_type`` FK to ContentType.

    For use in trees of inherited models, to be able to downcast
    parent instances to their child types.

    """
    real_type = models.ForeignKey(ContentType, editable=False)

    objects = PolymorphicManager()

    def save(self, *args, **kwargs):
        if not self.id:
            self.real_type = self._get_real_type()
        super(PolymorphicModel, self).save(*args, **kwargs)

    def _get_real_type(self):
        return ContentType.objects.get_for_model(type(self))

    def cast(self):
        return self.real_type.get_object_for_this_type(pk=self.pk)

    class Meta:
        abstract = True

### EXAMPLE MODELS ###

# Classified
  # Vehicle
    # Motor
    # Car
  # Fashion

### END OF EXAMPLE MODELS ###
class Classified(PolymorphicModel):
    name = models.CharField(max_length=200)


class Vehicle(Classified):
    year_assembled = models.CharField(max_length=5)


class Fashion(Classified):
    color = models.CharField(max_length=50)


class Motor(Vehicle):
    pass


class Car(Vehicle):
    pass