Ability to use `attr_protected` attributes as sign-in keys without triggering `ActiveModel::MassAssignmentSecurity::Error` when `ActiveRecord::Base.mass_assignment_sanitizer` is set to `:strict`. Fixes #1729.

app/controllers/devise/sessions_controller.rb

@@ -4,7 +4,7 @@ class Devise::SessionsController < DeviseController
 
   # GET /resource/sign_in
   def new
-    resource = build_resource
+    resource = build_resource(nil, :unsafe => true)
     clean_up_passwords(resource)
     respond_with(resource, serialize_options(resource))
   end

app/controllers/devise_controller.rb

@@ -67,10 +67,21 @@ MESSAGE
     instance_variable_set(:"@#{resource_name}", new_resource)
   end
 
-  # Build a devise resource.
-  def build_resource(hash=nil)
+  # Build a devise resource. 
+  # Assignment bypasses attribute protection when :unsafe option is passed
+  def build_resource(hash = nil, options = {})
     hash ||= params[resource_name] || {}
-    self.resource = resource_class.new(hash)
+
+    if options[:unsafe]
+      self.resource = resource_class.new.tap do |resource|
+        hash.each do |key, value|
+          setter = :"#{key}="
+          resource.send(setter, value) if resource.respond_to?(setter)
+        end
+      end
+    else
+      self.resource = resource_class.new(hash)
+    end
   end
 
   # Helper for use in before_filters where no authentication is required.

test/controllers/sessions_controller_test.rb

@@ -13,4 +13,20 @@ class SessionsControllerTest < ActionController::TestCase
     assert_equal 200, @response.status
     assert_template "devise/sessions/new"
   end
+  
+  test "#new doesn't raise mass-assignment exception even if sign-in key is attr_protected" do
+    request.env["devise.mapping"] = Devise.mappings[:user] 
+ 
+    ActiveRecord::Base.mass_assignment_sanitizer = :strict
+    User.class_eval { attr_protected :email }
+    
+    begin
+      assert_nothing_raised ActiveModel::MassAssignmentSecurity::Error do
+        get :new, :user => { :email => "allez viens!" }
+      end
+    ensure
+      ActiveRecord::Base.mass_assignment_sanitizer = :logger
+      User.class_eval { attr_accessible :email }
+    end
+  end
 end