Class based default value for field in Django model inheritance hierarchy

How can one accomplish class-based default value in following scheme? I mean, I would like to inherited classes set default value for "number" differently:

class OrderDocumentBase(PdfPrintable):
    number = models.PositiveIntegerField(default=self.create_number())

    @classmethod
    def create_number(cls):
        raise NotImplementedError

class Invoice(OrderDocumentBase):
    @classmethod
    def create_number(cls):
        return 1

class CreditAdvice(OrderDocumentBase):
    @classmethod
    def create_number(cls):
        return 2

I have looked at this stackoverflow question, but it doesn't address the same problem. The only thing I thought would work was overloading OrderDocumentBase's __init__ method like this:

def __init__(self, *args, **kwargs):
    """
    Overload __init__ to enable dynamic set of default to number
    """
    super(OrderDocumentBase, self).__init__(*args, **kwargs)
    number_field = filter(lambda x: x.name == 'number', self._meta.fields)[0]
    number = self.__class__.create_number()
    number_field.default = number

This works, but only partially and behaves quite wierdly. In admin interface, I can see the default being set only after second or latter page refresh. On first try, None is being set :(

Second possibility is redefinition of number field in each class, but that doesn't seem too much pretty. Is there any other way?

Can someone help?