In Ruby on Rails, when you want to get view image_path etc. from within a controller, you can use view_context.

■ Controller Example

#-*- encoding: utf-8 -*-
 
class BookController < ApplicationController
  def show
    view_context.image_path('book.jpg')
      # => "http://localhost:3000/images/book.jpg"
  end
end

■ application.rb

module MyApp
  class Application < Rails::Application
    config.action_controller.asset_host = "http://localhost:3000"
    ...(omitted)...
  end
end

That’s all.

【Reference】

・Rails のコントローラの中で View の画像URLを取得する方法 - それはBlog (How to Get View Image URLs from Within a Rails Controller - That’s Blog)

That’s all from the Gemba.