code: def on_change_check_out(self):
          '''
          When you change checkout or checkin it will check whether
          Checkout date should be greater than Checkin date
          and update dummy field
          -----------------------------------------------------------
          @param self: object pointer
          @return: raise warning depending on the validation
          '''
          if self.check_out and self.check_in:
              if self.check_out < self.check_in:
                  raise ValidationError(_('Checkout date should be greater \
                                           than Checkin date.')f on_change_check_out(self):
          '''
          When you change checkout or checkin it will check whether
          Checkout date should be greater than Checkin date
          and update dummy field
          -----------------------------------------------------------
          @param self: object pointer
          @return: raise warning depending on the validation
          '''
          if self.check_out and self.check_in:
              if self.check_out < self.check_in:
                  raise ValidationError(_('Checkout date should be greater \
                                           than Checkin date.'))
docs: When you change checkout or checkin it will check whether
          Checkout date should be greater than Checkin date
          and update dummy field
          -----------------------------------------------------------
          @param self: object pointer
          @return: raise warning depending on the validation