Topic: Setting Up Relationships

So I'm diving in on my first Rails project ever and I wanted to get some advice on how I'm planning on setting up my relationships with my database.

For a little background on the project, I want to have a certain amount of disciplines, each with their own classes.  Each trainer has their own classes and each user signs up for each class.  Each user also has a certain amount of credits that they must use in order to be a part of a certain class.  The users also have stats.

Here's how I plan on having it set up.  Since it's my first Rails endeavor I'm sure that you will have some insight into how I'm planning on doing it.  The only join table I plan on having is one for class enrollment that the users and the trainers share.  I'm almost positive I'm not setting up the correct relationship for the credits but who knows.

Any help would be greatly appreciated!