observers.rb

module MongoMapper
  module Observer
    extend ActiveSupport::Concern

    included do
      include ::ActiveModel::Observing
      class_eval do
        [
          :before_save,   :after_save,
          :before_create, :after_create,
          :before_update, :after_update
        ].each do |method|
          self.send method do |record|
            # record.class.changed
            record.class.notify_observers(method, record)
          end
        end
      end
    end
  end
end