Django: Check if foreign key attribute is set

I have the following model:

class A(models.Model):
    name = models.CharField(max_length=50)
    content_type = models.ForeignKey(ContentType)

This model is supposed to be the root model in some inheritance tree and content_type attribute is a kind of a hint about what type is really stored. Obviously, I should calculate content_type transparently upon A instance creation. I think, in __init__. But there is a problem - there are two main contexts in which A instances are created:

1. a = A(name='asdfdf') # here we must fill in content_type
2. by QuerySet machinery with *args tuple. In this case I shouldn't fill in content_type

So, I'm trying to write:

def __init__(self, *args, **kwargs):
    super(A, self).__init__(*args, **kwargs)
    if self.content_type is None: # << here is the problem
        self.content_type = ContentType.objects.get_for_model(self)

The thing is self.content_type is ReverseSingleRelatedObjectDescriptor instance with __get__ overriden so that it throws in case value is not set. Yes, I 开发者_StackOverflow中文版can do following:

def __init__(self, *args, **kwargs):
    super(A, self).__init__(*args, **kwargs)
    try: 
        self.content_type
    except Exception, v:
        self.content_type = ContentType.objects.get_for_model(self)

But I don't like it. Is there a more 'polite' way to check if the ForeignKey attribute is set?

Does it work if you examine self.content_type_id instead of self.content_type?