Hi,

First post, sorry if this is the wrong place or has been answered before.

I'm using ActiveRecord and I want to enforce mutual exclusivity on a 
has_and_belongs_to_many.

A concrete example: I have a User which can have one or more Roles 
(student, tutor, headmaster, administrator etc). However if a user is a 
student they cannot hold any other role. I was hoping to find something 
like validate_exclusive but that doesn't appear to exist. Can anyone 
suggest how I can implement this. Otherwise, is there  a better pattern 
I could use to model this?

Many thanks,

Toby.