Use scope instead of constraints to define routes.

It works the same with rails 3 and rails 4.

test/failure_app_test.rb

@@ -12,7 +12,9 @@ class FailureTest < ActiveSupport::TestCase
     routes = ActionDispatch::Routing::RouteSet.new
 
     routes.draw do
-      root to: 'foo#bar', constraints: { subdomain: 'sub' }
+      scope subdomain: 'sub' do
+        root to: 'foo#bar'
+      end
     end
 
     include routes.url_helpers

test/rails_app/config/routes.rb

@@ -66,7 +66,7 @@ Rails.application.routes.draw do
     devise_for :homebase_admin, class_name: "Admin", path: "homebase"
   end
 
-  constraints(subdomain: 'sub') do
+  scope(subdomain: 'sub') do
     devise_for :subdomain_users, class_name: "User", only: [:sessions]
   end