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

closes #3341

4 files changed, 27 insertions, 2 deletions

diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index 566b6af378..e1b68d9f26 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,5 +1,7 @@
 ## Rails 3.2.3 (unreleased) ##
 
+*   Allow to lazy load `default_form_builder` by passing a `String` instead of a constant. *Piotr Sarnacki*
+
 *   Fix #5632, render :inline set the proper rendered format. *Santiago Pastorino*
 
 *   Fix textarea rendering when using plugins like HAML. Such plugins encode the first newline character in the content. This issue was introduced in https://github.com/rails/rails/pull/5191 *James Coleman*
diff --git a/actionpack/lib/action_view/helpers/form_helper.rb b/actionpack/lib/action_view/helpers/form_helper.rb
index 252fe1ed03..a3409ee3c7 100644
--- a/actionpack/lib/action_view/helpers/form_helper.rb
+++ b/actionpack/lib/action_view/helpers/form_helper.rb
@@ -9,6 +9,7 @@ require 'active_support/core_ext/module/method_names'
 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/core_ext/string/inflections'
 
 module ActionView
   # = Action View Form Helpers
@@ -959,9 +960,14 @@ 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, block)
         end
+
+        def default_form_builder
+          builder = ActionView::Base.default_form_builder
+          builder.respond_to?(:constantize) ? builder.constantize : builder
+        end
     end
 
     class InstanceTag
diff --git a/actionpack/test/template/form_helper_test.rb b/actionpack/test/template/form_helper_test.rb
index 7eeede7087..3f459bad2e 100644
--- a/actionpack/test/template/form_helper_test.rb
+++ b/actionpack/test/template/form_helper_test.rb
@@ -2033,6 +2033,23 @@ 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)
diff --git a/railties/guides/source/configuring.textile b/railties/guides/source/configuring.textile
index 60034cdda5..3466e1c3f4 100644
--- a/railties/guides/source/configuring.textile
+++ b/railties/guides/source/configuring.textile
@@ -345,7 +345,7 @@ There are only a few configuration options for Action View, starting with three
 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.erb_trim_mode+ gives the trim mode to be used by ERB. It defaults to +'-'+. See the "ERB documentation":http://www.ruby-doc.org/stdlib/libdoc/erb/rdoc/ for more information.