Have we learned nothing in the last 10 years? Aside from the siren song of better performance, there are few other reasons to model your domain in the database.

Throwing business logic into stored procedures wily-nilly is a terrible idea for a lot of reasons. First, there is no standard method for testing stored procedures functions; you have to put a lot more effort into devising a test strategy for your database functions than your controller functions. In addition, you've now coupled the business logic with the database, and you cannot move your data around easily. And, lastly, if you have a large database, adding new functions and triggers will involve some downtime, too.

To sum up, don't replace your Rails app with a database unless you really know what you're doing.