ActiveAdmin on admin domain

admin_domain.rb

class AdminDomain

  def self.matches?(request)
    request.subdomain =~ /admin/
  end

end

routes.rb

YourApp::Application.routes.draw do

  # We'll constrain admin to only respond on admin.yoapp.com
  constraints(AdminDomain) do
    ActiveAdmin.routes(self)
    devise_for :admin_users, ActiveAdmin::Devise.config
  end

  # rest of yo'app here

end