Polymorphism in Django models

Question

I'm developing django application, and I have such a model structure

class Animal(models.Model):
    aul = models.ForeignKey(Aul)
    age = models.IntegerField()

    def __unicode__(self):
        return u'Animal'

class Sheep(Animal):
    wool = models.IntegerField()

    def __unicode__(self):
        return u'Sheep'

And I pass animal_set to template and output every object like this {{ animal }}. It outputs Animal, but I created objects of sheep type and want to use __unicode__ method of sheep not of animal.

Do polymorphism work in Django models? I have found several answers, but there are snippets of code to write inside models, but I'm interested in native solutions.

Answer 1

At the time of writing, Django latest version was 1.2

But it needs some additional elements to work.

You need to assign a custom models.Manager object for each animal model which will call its own custom QuerySet object.

Basically, instead of returning Animal instances (this is what you get), SubclassingQuerySet calls as_leaf_class() method to check if item's model is Animal or not - if it is, then just return it, otherwise perform search in its model context. Thats it.

#models.py
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.as_leaf_class()
        return result

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


class AnimalManager(models.Manager):
    def get_query_set(self):  # Use get_queryset for Django >= 1.6
        return SubclassingQuerySet(self.model)


class Animal(models.Model):
    name = models.CharField(max_length=100)
    content_type = models.ForeignKey(ContentType, editable=False, null=True)
    objects = AnimalManager()

    def __unicode__(self):
        return "Animal: %s" % (self.name)

    def save(self, *args, **kwargs):
        if not self.content_type:
            self.content_type = ContentType.objects.get_for_model(self.__class__)
        super(Animal, self).save(*args, **kwargs)

    def as_leaf_class(self):
        content_type = self.content_type
        model = content_type.model_class()
        if model == Animal:
            return self
        return model.objects.get(id=self.id)


class Sheep(Animal):
    wool = models.IntegerField()
    objects = AnimalManager()

    def __unicode__(self):
        return 'Sheep: %s' % (self.name)

Testing:

>>> from animals.models import *
>>> Animal.objects.all()
[<Sheep: Sheep: White sheep>, <Animal: Animal: Dog>]
>>> s, d = Animal.objects.all()
>>> str(s)
'Sheep: White sheep'
>>> str(d)
'Animal: Dog'
>>>