gistfile1.rb

class Order
  include Mongoid::Document
  embeds_one :cart
end

class Cart
  include Mongoid::Document
  embedded_in :order
  embeds_many :cart_items
end

class CartItem
  include Mongoid::Document
  embedded_in :cart
  field :title, :type => String
end

class OrderAuditObserver < Mongoid::Observer
  observe :order
  
  def around_update(resourse)
    changes = resourse.changes.clone
    yield
    #case resourse.class
    #when Order
    #end
  end
end