Python, referencing a sub-class

Hello experts,

I am having a problem that I hope there is an solution to.

I have a base class called Document. I then have few sub-classes of Document (Book, Magazine, ...). My problem lies in the fact that a Book has an author attribute but a Magazine doesn't. So when I do:

documents = bookModel.Document.objects.all()

for document in documents:
    print document.author

I get nothing since the base class doesn't have an author attribute.

Do these instanced of a base class not know they are really  implemented as sub-classes ?

I hope I am being clear enough