Lazy load `default_form_builder` if it's passed as a string

closes #3341

3 files changed, 26 insertions, 1 deletions

diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index 9f1c44ca66..e827f935c3 100644
--- a/actionpack/CHANGELOG.md
+++ b/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
diff --git a/actionpack/lib/action_view/helpers/form_helper.rb b/actionpack/lib/action_view/helpers/form_helper.rb
index 8e7224937d..67f2abe509 100644
--- a/actionpack/lib/action_view/helpers/form_helper.rb
+++ b/actionpack/lib/action_view/helpers/form_helper.rb
@@ -10,6 +10,7 @@ require 'active_support/core_ext/object/blank'
 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 @@ module ActionView
             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
diff --git a/actionpack/test/template/form_helper_test.rb b/actionpack/test/template/form_helper_test.rb
index a714264909..4018c96c99 100644
--- a/actionpack/test/template/form_helper_test.rb
+++ b/actionpack/test/template/form_helper_test.rb
@@ -2169,6 +2169,23 @@ class FormHelperTest < ActionView::TestCase
     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)