active_decorator.rb

class ActiveDecorator

  include ::Rails.application.routes.url_helpers

  def default_url_options
    ::ActionMailer::Base.default_url_options
  end

  attr_reader :decorated

  def initialize(decorated)
    @decorated = decorated
  end

end

thing_decorator.rb

module Api

  class ThingDecorator < ActiveDecorator

    def as_json(options = {})
      thing = decorated

      {
        :id   => thing.id,
        :name => thing.name,
      }
    end

  end

end

usage.rb

thing = Thing.find(1)
decorator = Api::ThingDecorator.new(thing)
render :json => decorator.as_json