class PolyMorphicModel(models.Model):
    class meta:
        proxy = True
    __type  = models.CharField( max_length = 100 )
    def save(self, *args, **kwargs):
        """
        set the correct type before saving
        """
        self.__type=self.__class__.__name__.lower()
        if (self.__type == 'vehicle'):
            raise NotImplementedError()
        super(Vehicle, self).save(*args, **kwargs)

    def get_child_object(self):
        """
        Returns the correct child object for an instance
        """
        if (self.__type == 'vehicle'):
            raise NotImplementedError()
        return getattr(self, self.__type)

class Vehicle(PolyMorphicModel):
    color = models.CharField( max_length = 100 )

class Car(Vehicle):
    model = models.CharField( max_length = 100 )

class Boat(Vehicle):
    captain = models.CharField( max_length = 100 )

audi = Car(model='audi', color='red')
audi.save()
car = Vehicle.objects.get(id=audi.id).get_child_object()
assert car.color=='red'
assert car.model=='audi'

ship = Boat(color = 'black', captain = 'Hook')
ship.save()
boat = Vehicle.objects.get(id=ship.id).get_child_object()
assert boat.captain == 'Hook'
assert boat.color == 'black'