gistfile1.rb

# routes
namespace :api, path: '/' do
  constraints :subdomain => 'api' do
    get 'users' => 'users#index'
  end
end
  
# url_for

link_to "Api User", api_users_path
#=> yourdomain.com/users

link_to "Api User", api_users_path(subdomain: "api")
#=> api.yourdomain.com/users