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GitHub User: mstarkman
Just before the episode was release I started working on a gem to solve this very problem. It's called CanBe. I've also implemented the code from this episode to use my gem here.
Take a look at my blog post here http://createbycoding.com/2012/02/getting-backbone-js-to-sync-with-rails-and-mongoid/. I had a similar issue a few months ago.