DynamicForm holds a few helper methods to help you deal with your Rails3 models. It includes the stripped out methods from Rails 2; error_message_on and error_messages_for. It also brings in the functionality of the custom-err-messages plugin, which provides more flexibility over your model error messages.
Gemfile gem 'dynamic_form', '~> 1.1.2'
install gem install dynamic_form -v 1.1.2