Lazy load default_form_builder if it's passed as a string

closes rails#3341

actionpack/CHANGELOG.md

@@ -1,5 +1,7 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Allow to lazy load `default_form_builder` by passing a `String` instead of a constant. *Piotr Sarnacki*
+
 *   Session arguments passed to `process` calls in functional tests are now merged into
     the existing session, whereas previously they would replace the existing session.
     This change may break some existing tests if they are asserting the exact contents of

actionpack/lib/action_view/helpers/form_helper.rb

@@ -10,6 +10,7 @@
 require 'active_support/core_ext/string/output_safety'
 require 'active_support/core_ext/array/extract_options'
 require 'active_support/deprecation'
+require 'active_support/core_ext/string/inflections'
 
 module ActionView
   # = Action View Form Helpers
@@ -1039,9 +1040,14 @@ def instantiate_builder(record_name, record_object, options)
             object_name = ActiveModel::Naming.param_key(object)
           end
 
-          builder = options[:builder] || ActionView::Base.default_form_builder
+          builder = options[:builder] || default_form_builder
           builder.new(object_name, object, self, options)
         end
+
+        def default_form_builder
+          builder = ActionView::Base.default_form_builder
+          builder.respond_to?(:constantize) ? builder.constantize : builder
+        end
     end
 
     class FormBuilder

actionpack/test/template/form_helper_test.rb

@@ -2169,6 +2169,23 @@ def test_default_form_builder
     ActionView::Base.default_form_builder = old_default_form_builder
   end
 
+  def test_lazy_loading_default_form_builder
+    old_default_form_builder, ActionView::Base.default_form_builder =
+      ActionView::Base.default_form_builder, "FormHelperTest::LabelledFormBuilder"
+
+    form_for(@post) do |f|
+      concat f.text_field(:title)
+    end
+
+    expected = whole_form('/posts/123', 'edit_post_123', 'edit_post', :method => 'patch') do
+      "<label for='title'>Title:</label> <input name='post[title]' type='text' id='post_title' value='Hello World' /><br/>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  ensure
+    ActionView::Base.default_form_builder = old_default_form_builder
+  end
+
   def test_fields_for_with_labelled_builder
     output_buffer = fields_for(:post, @post, :builder => LabelledFormBuilder) do |f|
       concat f.text_field(:title)

guides/source/configuring.textile

@@ -362,7 +362,7 @@ h4. Configuring Action View
 Proc.new { |html_tag, instance| %Q(<div class="field_with_errors">#{html_tag}</div>).html_safe }
 </ruby>
 
-* +config.action_view.default_form_builder+ tells Rails which form builder to use by default. The default is +ActionView::Helpers::FormBuilder+.
+* +config.action_view.default_form_builder+ tells Rails which form builder to use by default. The default is +ActionView::Helpers::FormBuilder+. If you want your form builder class to be loaded after initialization (so it's reloaded on each request in development), you can pass it as a +String+
 
 * +config.action_view.logger+ accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then used to log information from Action View. Set to +nil+ to disable logging.