Best way to do default values

I'm working on a Role-based user authentication model, where a user
habtm roles, and roles habtm rights. My problem is that I know that I
need specific Role instances right off the bat, so how do i go about
ensuring they're in the system. I know I could do this in a migration,
but it doesn't seem like the right approach, as it's a bit constrictive.

I've also thought of defining class variables with the default values.
Anyone have any thoughts/experience doing something similar?