django session key changing upon authentication

I have a Django app which records users' product choices for both authenticated users. My intention is to use the request.session.session_key variable to associate anonymous data with a user if they decide to register later, a la this post:

Django storing anonymous user data

However, it seems that the session key changes when the user logs in/ registers so the session key can no longer be associated with the user. Is this the correct behaviour of the Django session framework. Is there a solid way to achieve the functionality I'm looking for?

Any help much appreciated.