Using helpers in ActionMailer

How do I access helpers within ActionMailer from ones I have created?

I have tried: How to use my view helpers in my ActionMailer views? for my own created h开发者_如何学运维elper but didn't work. Any advise?

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This has worked for me in the past in Rails 3:

class OrderMailer < ActionMailer::Base
  helper :application, :orders, :products
  ...
end

My custom helpers are obviously orders and products.