active_record_touch.rb

class ActiveRecord::Base
  descendants.each do |klass|
    klass.singleton_class.instance_eval do
      attr_accessor :touches, :touched_by
    end
    klass.touches = []
    klass.touched_by = []

    klass.reflections.values.each do |reflection|
      if reflection.options && reflection.options[:touch]
        toucher = klass
        touched = Kernel.const_get(reflection.class_name)
        toucher.touches << touched
        touched.touched_by << toucher
      end
    end

    klass.instance_eval do
      def updates
        unless self.touches
          []
        else
          direct = self.touches
          indirect = direct.map &:updates
          (direct + indirect).flatten.uniq
        end
      end

      def updated_by
        unless self.touched_by
          []
        else
          direct = self.touched_by
          indirect = direct.map &:updated_by
          (direct + indirect).flatten.uniq
        end
      end
    end
  end
end