Need two fields together to be unique

Say I have a model like this:

class Book(models.Model):

    user = models.ForeignKey(User)
    book_isbn = models.CharField()

I would like the combination of the book and user fields to be unique. That is different users could have the same book, but a specific user could only have the book once in their list of books.

I thought unique_together would be the perfect solution, but them read it can't be used if any of the fields have a FK. What's the best way to achieve what I want?

I'm currently using multiple django models with unique_together clause which contained foreign keys and I never had any problems. I currently using django 1.2.