Fluent NHibernate Many-to-many mapping with auto-generated pk instead of composite key

I’ve been using Fluent NHibernate for the past couple of weeks. This is my first project using NHibernate, and to lessen the configuration time, I decided to use Fluent NHibernate.

I really like the Fluent NHibernate interface. However, I immediately began to have trouble using join tables in my MembershipProvider implementation.
The table structure of the problematic code is shown in the image above.