Primary key on HABTM join table now raises an exception

Hello all. Most of us have created a habtm relationship at some point, and forgotten to remove the default primary key from the join table. It works at first, tests usually pass, but eventually you get cryptic duplicate primary key errors from the database.

My fix raises an exception any time you try to create a habtm association. It rails right away, loudly, and with a clear message letting you know what you've done wrong. I'd really appreciate reviews and feedback. Here's the ticket: