Nope, there's no schema change. If you look below, you will see a function 'username' defined. That allows me to create a field from a different one (user.username). Then there is a magical line that allows me to control how the sorting works (username.admin_order_field = 'user__username').

I of course only figured this out by googling for examples. In general, there's far too much magic in both python and django for my taste. :)