Polymorphic Django Models

models.py

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

class PolymorphicMetaclass(ModelBase):
    def __new__(cls, name, bases, dct):
        def save(self, *args, **kwargs):
            if(not self.content_type):
                self.content_type = ContentType.objects.get_for_model(self.__class__)
            models.Model.save(self, *args, **kwargs)

        def downcast(self):
            model = self.content_type.model_class()
            if (model == self.__class__):
                return self
            return model.objects.get(id=self.id)

        if issubclass(dct.get('__metaclass__', type), PolymorphicMetaclass):
            dct['content_type'] = models.ForeignKey(ContentType, editable=False, null=True)
            dct['save'] = save
            dct['downcast'] = downcast

        return super(PolymorphicMetaclass, cls).__new__(cls, name, bases, dct)

class DowncastMetaclass(PolymorphicMetaclass):
    def __new__(cls, name, bases, dct):
        dct['objects'] = DowncastManager()
        return super(DowncastMetaclass, cls).__new__(cls, name, bases, dct)

class DowncastManager(models.Manager):
    def get_query_set(self):
        return DowncastQuerySet(self.model)

class DowncastQuerySet(QuerySet):
    def __getitem__(self, k):
        result = super(DowncastQuerySet, self).__getitem__(k)
        if isinstance(result, models.Model) :
            return result.downcast()
        else :
            return result
    def __iter__(self):
        for item in super(DowncastQuerySet, self).__iter__():
            yield item.downcast()

# Using them 

class Content(models.Model):
    __metaclass__ = DowncastMetaclass

    owner = models.ForeignKey(User, related_name='owned_content')
    title = models.TextField(max_length=255)

class AudioFile(Content):
    external_file = models.FileField(upload_to='audio',editable=False,help_text='Should be a m3u8 playlist file for now.')

    def __unicode__(self):
        return self.title

    def save(self, force_insert=False, force_update=False, using=None):
        # I want to do something here but I can't figure out how.

        return super(AudioFile, self).save(force_insert, force_update, using)

If I'm understanding what you're trying to do correctly, you're much better of using an abstract base class.