I've seen it somewhere.. and I've added the 'date_created' thing that you can't access when you're doing it all RESTy.

mongoid_json_patch.rb

# Adds "id" and "date_created" to the mongoid document.
# "id" => "_id"
# "date_created" => "_id.generation_time"
# YEAH. thats it.
module Mongoid
  module Document
    def as_json(options={})
      attrs = super(options)
      attrs["id"] = self.persisted? ? self._id : nil
      attrs["date_created"] = self.persisted? ? self._id.generation_time : nil
      attrs
    end
  end
end