def save_response(answer_sheet)
      unless @answers.nil?
        for answer in @answers
          if answer.is_a?(Answer)
            answer.answer_sheet_id = answer_sheet.id
            answer.save!
          end
        end
      end
      
      # remove others
      unless @mark_for_destroy.nil?
        for answer in @mark_for_destroy
          answer.destroy
        end
        @mark_for_destroy.clear
      end
    rescue TypeError
      raise answer.inspect
    end
def save_response(answer_sheet)
      unless @answers.nil?
        for answer in @answers
          if answer.is_a?(Answer)
            answer.answer_sheet_id = answer_sheet.id
            answer.save!
          end
        end
      end
      
      # remove others
      unless @mark_for_destroy.nil?
        for answer in @mark_for_destroy
          answer.destroy
        end
        @mark_for_destroy.clear
      end
    rescue TypeError
      raise answer.inspect
    end
save this question's @answers to database