instance method retrieve_autoindex

# File actionpack/lib/action_view/helpers/form_helper.rb, line 1133
      def retrieve_autoindex(pre_match)
        object = self.object || @template_object.instance_variable_get("@#{pre_match}")
        if object && object.respond_to?(:to_param)
          object.to_param
        else
          raise ArgumentError, "object[] naming but object param and @object var don't exist or don't respond to to_param: #{object.inspect}"
        end
      end