Simple Rails' Rack example.

simple_rails_rack_example.rb

# This file is used by Rack-based servers to start the application.
require ::File.expand_path('../config/environment', __FILE__)

class MyMiddleware
  def initialize(app)
    @app = app
  end

  def call(env)
    puts 'foobar'
    @app.call(env)
  end
end

# rackup config.ru

use MyMiddleware
run Rails.application