Simple HTTP OPTIONS response middleware for Rails

options_response.rb

module Rack
  class OptionsResponse
    def initialize(app)
      @app = app
    end

    def call(env)
      request = Rack::Request.new(env)
      if request.options?
        generate_response(request)
      else
        @app.call(env)
      end
    end

    def generate_response(request)
      recognizable_methods = %w(GET PUT POST PATCH DELETE)
      allowed_methods = recognizable_methods.select do |method|
        begin
          Rails.application.routes.recognize_path(request.path, method: method)
        rescue ActionController::RoutingError
          false
        end
      end
      return [404, {}, []] if allowed_methods.empty?
      [204, { "Allow" => %w(HEAD).concat(allowed_methods).join(",") }, []]
    end
  end
end

Rails.configuration.middleware.use Rack::OptionsResponse