Inspired by another post: I know many Rails developers who know and can use Rails very well but don't really know Ruby. I recently worked with a seasoned Rails developer who didn't realise that attr_accessor is just a class method called within the class, (a.k.a class macro). The same went for Rails routes. He was under the impression it was some kind of Rails 'magic' and I suppose he was right in a sense but -under the magic- it;s just plain, old Ruby.
My point is: Do you think that devs should learn Ruby first? Do you think it doesn't make a difference as long as they know Rails? If you could go back in time, would you have learned Ruby before Rails?
My take is this: learning Ruby makes people better Rails developers. I strongly recommend, if you're contemplating doing Rails for the first time, to dedicate some time to get up to speed with Ruby first. It will save a lot of pain and hassle in the long run.