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*
+
 *   Remove the leading \n added by textarea on assert_select. *Santiago Pastorino*
 
 *   Changed default value for `config.action_view.embed_authenticity_token_in_remote_forms`

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
@@ -1026,9 +1027,14 @@ 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

@@ -2119,6 +2119,23 @@ 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.