Rearrangeable ordered list with Ruby on Rails
If you want to roll your own, the answers to this question might be helpful:
Best way to save a ordered List to the Database while keeping the ordering
Seems like the consensus is to just use an order field and re-order them all on save. You can mitigate the amount of database work if you allow the user to do the reordering interactively client side, and just send the final list to save back to the server.