Hi,
it's been some time since your post, but I'll respond to it anyway because maybe you're still looking for a solution.
I'd suggest you to use Riff. It will let you compare two AR objects with each other. Before you apply the form data to your object, you could clone it to a @dbrecord-var and afterward compare both objects with Riff. You can find more on riff here: http://wiki.rubyonrails.org/rails/pages/Riff+Plugin
Greetings, Christoph