Well, simply enough, it's kind of a difficult problem to solve. The way most rails apps do it, is deployment with Capistrano or similar. There's also Fabric which can interface pretty well with rails deployment as well. Honestly the methodology I've seen under most deployment situations is far, far from idempotent and is a bit terrible in this respect.

There are lots of tools within Rails and Capistrano that hopefully get you to a state approaching 'idempotent' deployments, but they fairly often aren't.