That's true and all, but Rails and Django were built to run on SQL dbs right out of the box. Getting them to work with MongoDB takes some work, and I don't think it's a diversion that someone just starting learning frameworks should try to deal with.

As for migrations, I don't think he's gonna be dealing with anything too complicated schema-wise. Django South will write your migrations for you. Maybe Rails has something like that? But honestly, I'd recommend using SQLite and just starting a new file whenever he really felt the need to alter the schema.