Be more friendly if the user goes ahead and adds devise_for :users before defining the model.

lib/devise/rails/routes.rb

@@ -93,6 +93,11 @@ module ActionDispatch::Routing
 
         begin
           raise_no_devise_method_error!(mapping.class_name) unless mapping.to.respond_to?(:devise)
+        rescue NameError => e
+          raise unless mapping.class_name == resource.to_s.classify
+          warn "[WARNING] You provided devise_for #{resource.inspect} but there is " <<
+            "no model #{mapping.class_name} defined in your application"
+          next
         rescue NoMethodError => e
           raise unless e.message.include?("undefined method `devise'")
           raise_no_devise_method_error!(mapping.class_name)

test/rails_app/config/routes.rb

@@ -7,8 +7,8 @@ Rails::Application.routes.draw do
 
   devise_for :users
   devise_for :admin, :path => "admin_area", :controllers => { :sessions => "sessions" }, :skip => :passwords
-  devise_for :accounts, :singular => "manager", :path_prefix => ":locale",
-    :class_name => "User", :path_names => {
+  devise_for :accounts, :singular => "manager", :path_prefix => ":locale", :class_name => "User",
+    :path_names => {
       :sign_in => "login", :sign_out => "logout",
       :password => "secret", :confirmation => "verification",
       :unlock => "unblock", :sign_up => "register",