usage of class inheritance in django for polymorphism?

I need your help in the following matter :

in my django models, the following classes exist :

class QItem(models.Model)


class Question(QItem)


class QuestionSet(QItem):
    items = models.ManyToManyField(QItem, blank=True, null=True, through='Ordering', related_name="contents")


class Ordering(models.Model):
    item = models.ForeignKey(QItem)
    qset = models.ForeignKey(QuestionSet, related_name="questionSet")
    order = models.IntegerField(unique=True)

So QuestionSet should be able to contain both Question objects, and other QuestionSet objects, in the order specified in Ordering.order.

When performing

o1 = Ordering(item=q, qset=q1)

where q1 is a Question object, I get

ValueError:开发者_StackOverflow社区 Cannot assign "[<Question: q1>]": "Ordering.item" must be a "QItem" instance.

What is the correct way to get this functionality? (to be able to treat both questions and questionsets at the same time?) Is it even possible?

Although I haven't tested it yet, seems that a possible resolution might lie in this :

http://bserve.webhop.org/wiki/django_polymorphic/doc

and it should be as easy as the following (copied from above link for convenience)

from polymorphic import PolymorphicModel

class ModelA(PolymorphicModel):
    field1 = models.CharField(max_length=10)

class ModelB(ModelA):
    field2 = models.CharField(max_length=10)

class ModelC(ModelB):
field3 = models.CharField(max_length=10)