Rails middleware to enforce subdomain requests

subdomain_enforcer.rb

# app/middleware/subdomain_enforcer.rb
class SubdomainEnforcer
  def initialize(app)
    @app = app
  end

  def call(env)
    @request = Rack::Request.new(env)

    if @request.params['sbd'].present?
      new_location = get_new_location(@request.params['sbd'])

      log_redirect(new_location)

      [301, { 'Location' => new_location }, ['Moved Permanently']]
    else
      @status, @headers, @response = @app.call(env)
      [@status, @headers, self]
    end
  end

  def each(&block)
    @response.each(&block)
  end

  private

  def get_new_location(subdomain)
    uri = URI(@request.url)
    uri.host = "#{subdomain}.#{uri.host}"

    params = CGI.parse(uri.query || '')
    params.delete('sbd')
    uri.query = URI.encode_www_form(params)

    uri.to_s.gsub(/\?$/, '')
  end

  def log_redirect(url)
    Rails.logger.info "[sans_subdomain_link] method=#{@request.request_method} url=#{@request.url} redirect_to=#{url}"
  end
end

subdomain_enforcer_test.rb

# test/middleware/subdomain_enforcer_test.rb
require 'test_helper'
require 'rack'

class SubdomainEnforcerTest < ActiveSupport::TestCase

  setup do
    @app = lambda do |params|
      [200, {}, ""]
    end
  end

  test 'redirects to url with subdomain with params if sbd param is present' do
    request = Rack::MockRequest.env_for "https://workable.com/backend?sbd=winterfell&force_desktop=true"

    response = SubdomainEnforcer.new(@app).call(request)

    assert_equal 301, response[0]
    assert_equal({ 'Location' => 'https://winterfell.workable.com/backend?force_desktop=true' }, response[1])
    assert_equal(['Moved Permanently'], response[2])
  end

  test 'redirects to url with subdomain if sbd param is present' do
    request = Rack::MockRequest.env_for "https://workable.com/backend?sbd=winterfell"

    response = SubdomainEnforcer.new(@app).call(request)

    assert_equal 301, response[0]
    assert_equal({ 'Location' => 'https://winterfell.workable.com/backend' }, response[1])
    assert_equal(['Moved Permanently'], response[2])
  end

  test 'forwards request if sbd param is missing' do
    request = Rack::MockRequest.env_for "https://workable.com/backend?force_desktop=true"

    response = SubdomainEnforcer.new(@app).call(request)

    assert_equal 200, response[0]
  end
end