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

closes #3341

1 files changed, 7 insertions, 1 deletions

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