Allow form_for and fields_for to work with indexed form inputs. [Jeremy Kemper, Matt Lyon]

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@4613 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

1 files changed, 5 insertions, 1 deletions

diff --git a/actionpack/lib/action_view/helpers/form_helper.rb b/actionpack/lib/action_view/helpers/form_helper.rb
index eef7d1f567..43185e3a0c 100644
--- a/actionpack/lib/action_view/helpers/form_helper.rb
+++ b/actionpack/lib/action_view/helpers/form_helper.rb
@@ -238,7 +238,11 @@ module ActionView
         @template_object, @local_binding = template_object, local_binding
         @object = object
         if @object_name.sub!(/\[\]$/,"")
-          @auto_index = @template_object.instance_variable_get("@#{Regexp.last_match.pre_match}").id_before_type_cast
+          if object ||= @template_object.instance_variable_get("@#{Regexp.last_match.pre_match}") and object.respond_to?(:id_before_type_cast)
+            @auto_index = object.id_before_type_cast
+          else
+            raise ArgumentError, "object[] naming but object param and @object var don't exist or don't respond to id_before_type_cast: #{object.inspect}"
+          end
         end
       end