The Mongoid model would be
class User
include Mongoid::Document
field :first, type: String
field :last, type: String
end
There's the issue of mapping from Mongoid/MongoDB "_id" to Backbone "id". The choices to address this are summarized in several discussions that can be Googled, an example is http://dzello.com/blog/2011/12/24/tame-the-mongoid-id-field-in-your-rails-and-backbone-js-app/
Different people have chosen either of the two solutions, telling Backbone to use "_id" or rewriting #to_json in Mongoid to "id". Either will work, you may want to experiment to see what you prefer.