django model subclassing: get the subclass by querying the superclass

Question

The following code is given:

class BaseMedium(models.Model):
    title = models.CharField(max_length=40)
    slug = models.SlugField()

class A(BaseMedium):
    url = models.URLField()

class B(BaseMedium):
    email = models.EmailField()

I now want to query every BaseMedium.

 b = BaseMedium.objects.all()

How do I print every information including the subclass fields without knowing what the subclass type is?

b[0].a would print the information if b[0] is actually related to an A instance but if it's related to B it would print an DoesNotExist Exception. This makes sense but I'd like to have a common variable or method that returns the related object. Maybe my Database layout isn't really great to query that way if so I'd be glad if you'd recommend a better layout. I thought about using a GenericForeignKey

class Generic(models.Model):
    basemedium = models.ForeignKey('BaseMedium')
    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    object = generic.GenericForeignKey('content_type', 'object_id')

but this solution seems to be to complicated and I think you guys have better solutions.

Answer 1

The only way to do this is to explicitly store on the base model what type it is. So have a derived_type (or whatever) field on BaseMedium, and set it on save. Then you can have a get_derived_type method:

def get_derived_type(self):
    if self.derived_type ==  'A':
        return self.a
    elif self.derived_type == 'B':
        return self.b

and so on.

Answer 2

Thanks mr. Roseman for your reply. I developed your idea a bit further. Here is what I came up with:

def related_object(self, default_pointer_name='_ptr'):
        models = [A,B] #models
        object = None

        argument = '%s%s' %(self.__class__.__name__.lower(), default_pointer_name)
        query = { argument : self}

        for model in models:
            try:
                object = model.objects.get(**query)
            except model.DoesNotExist:
                pass
            else:
                return object

        if object == None:
            raise RelatedObjectException
        return object

This is a method used by BaseMedium.