module Mongoid::Touchable

def define_relation_touch_method(name, association)
  relation_classes = if association.polymorphic?
                       association.send(:inverse_association_classes)
                     else
                       [ association.relation_class ]
                     end
  relation_classes.each { |c| c.send(:include, InstanceMethods) }
  method_name = "touch_#{name}_after_create_or_destroy"
  association.inverse_class.class_eval do
    define_method(method_name) do
      without_autobuild do
        if relation = __send__(name)
          if association.touch_field
            # Note that this looks up touch_field at runtime, rather than
            # at method definition time.
            relation.touch association.touch_field
          else
            relation.touch
          end
        end
      end
    end
  end
  method_name.to_sym
end

def define_touchable!(association)
  name = association.name
  method_name = define_relation_touch_method(name, association)
  association.inverse_class.tap do |klass|
    klass.after_save method_name
    klass.after_destroy method_name
    klass.after_touch method_name
  end
end