diff --git a/actionpack/lib/action_view/helpers/form_helper.rb b/actionpack/lib/action_view/helpers/form_helper.rb
index 94dc25eb85..43dbedc448 100644
--- a/actionpack/lib/action_view/helpers/form_helper.rb
+++ b/actionpack/lib/action_view/helpers/form_helper.rb
@@ -304,12 +304,12 @@ module ActionView
           object_name = record_or_name_or_array
         when Array
           object = record_or_name_or_array.last
-          object_name = options[:as] || ActiveModel::Naming.singular(object)
+          object_name = options[:as] || ActiveModel::Naming.param_key(object)
           apply_form_for_options!(record_or_name_or_array, options)
           args.unshift object
         else
           object = record_or_name_or_array
-          object_name = options[:as] || ActiveModel::Naming.singular(object)
+          object_name = options[:as] || ActiveModel::Naming.param_key(object)
           apply_form_for_options!([object], options)
           args.unshift object
         end
@@ -539,7 +539,7 @@ module ActionView
           object_name = record
         else
           object = record
-          object_name = ActiveModel::Naming.singular(object)
+          object_name = ActiveModel::Naming.param_key(object)
         end
 
         builder = options[:builder] || ActionView::Base.default_form_builder
@@ -1168,11 +1168,11 @@ module ActionView
           end
         when Array
           object = record_or_name_or_array.last
-          name = "#{object_name}#{index}[#{ActiveModel::Naming.singular(object)}]"
+          name = "#{object_name}#{index}[#{ActiveModel::Naming.param_key(object)}]"
           args.unshift(object)
         else
           object = record_or_name_or_array
-          name = "#{object_name}#{index}[#{ActiveModel::Naming.singular(object)}]"
+          name = "#{object_name}#{index}[#{ActiveModel::Naming.param_key(object)}]"
           args.unshift(object)
         end
 
diff --git a/actionpack/test/lib/controller/fake_models.rb b/actionpack/test/lib/controller/fake_models.rb
index 37b200d57a..c4127ee699 100644
--- a/actionpack/test/lib/controller/fake_models.rb
+++ b/actionpack/test/lib/controller/fake_models.rb
@@ -149,3 +149,18 @@ class Author < Comment
   attr_accessor :post
   def post_attributes=(attributes); end
 end
+
+module Blog
+  def self._railtie
+    self
+  end
+
+  class Post < Struct.new(:title, :id)
+    extend ActiveModel::Naming
+    include ActiveModel::Conversion
+
+    def persisted?
+      id.present?
+    end
+  end
+end
diff --git a/actionpack/test/template/form_helper_test.rb b/actionpack/test/template/form_helper_test.rb
index ec57b6a2ab..97a08d45ba 100644
--- a/actionpack/test/template/form_helper_test.rb
+++ b/actionpack/test/template/form_helper_test.rb
@@ -75,6 +75,8 @@ class FormHelperTest < ActionView::TestCase
     @post.body        = "Back to the hill and over it again!"
     @post.secret      = 1
     @post.written_on  = Date.new(2004, 6, 15)
+
+    @blog_post = Blog::Post.new("And his name will be forty and four.", 44)
   end
 
   Routes = ActionDispatch::Routing::RouteSet.new
@@ -675,6 +677,21 @@ class FormHelperTest < ActionView::TestCase
     assert_dom_equal expected, output_buffer
   end
 
+  def test_form_for_with_isolated_namespaced_model
+    form_for(@blog_post) do |f|
+      concat f.text_field :title
+      concat f.submit('Edit post')
+    end
+
+    expected =
+      "<form accept-charset='UTF-8' action='/posts/44' method='post'>" +
+      snowman +
+      "<label for='post_title'>The Title</label>" +
+      "<input name='post[title]' size='30' type='text' id='post_title' value='And his name will be forty and four.' />" +
+      "<input name='commit' id='post_submit' type='submit' value='Edit post' />" +
+      "</form>"
+  end
+
   def test_form_for_with_symbol_object_name
     form_for(@post, :as => "other_name", :html => { :id => 'create-post' }) do |f|
       concat f.label(:title, :class => 'post_title')
diff --git a/railties/test/railties/engine_test.rb b/railties/test/railties/engine_test.rb
index 56f9eae8ca..73d9b61719 100644
--- a/railties/test/railties/engine_test.rb
+++ b/railties/test/railties/engine_test.rb
@@ -360,7 +360,7 @@ module RailtiesTest
       assert_equal "It's a bar.", response[2].body
     end
 
-    test "namespaced engine should include only its own routes and helpers" do
+    test "isolated engine should include only its own routes and helpers" do
       @plugin.write "lib/bukkits.rb", <<-RUBY
         module Bukkits
           class Engine < ::Rails::Engine
@@ -478,5 +478,68 @@ module RailtiesTest
       response = AppTemplate::Application.call(env)
       assert_equal "/bukkits/posts/1", response[2].body
     end
+
+    test "isolated engine should avoid namespace in names if that's possible" do
+      @plugin.write "lib/bukkits.rb", <<-RUBY
+        module Bukkits
+          class Engine < ::Rails::Engine
+            namespace Bukkits
+          end
+        end
+      RUBY
+
+      @plugin.write "app/models/bukkits/post.rb", <<-RUBY
+        module Bukkits
+          class Post
+            extend ActiveModel::Naming
+            include ActiveModel::Conversion
+            attr_accessor :title
+
+            def to_param
+              "1"
+            end
+
+            def persisted?
+              false
+            end
+          end
+        end
+      RUBY
+
+      app_file "config/routes.rb", <<-RUBY
+        AppTemplate::Application.routes.draw do
+          mount Bukkits::Engine => "/bukkits", :as => "bukkits"
+        end
+      RUBY
+
+      @plugin.write "config/routes.rb", <<-RUBY
+        Bukkits::Engine.routes.draw do
+          scope(:module => :bukkits) do
+            resources :posts
+          end
+        end
+      RUBY
+
+      @plugin.write "app/controllers/bukkits/posts_controller.rb", <<-RUBY
+        class Bukkits::PostsController < ActionController::Base
+          def new
+          end
+        end
+      RUBY
+
+      @plugin.write "app/views/bukkits/posts/new.html.erb", <<-RUBY
+          <%= form_for(Bukkits::Post.new) do |f| %>
+            <%= f.text_field :title %>
+          <% end %>
+      RUBY
+
+      add_to_config("config.action_dispatch.show_exceptions = false")
+
+      boot_rails
+
+      env = Rack::MockRequest.env_for("/bukkits/posts/new")
+      response = AppTemplate::Application.call(env)
+      assert response[2].body =~ /name="post\[title\]"/
+    end
   end
 end