code: def check_in_out_dates(self):
          
          if self.checkout and self.checkin:
              if self.checkin < self.date_order:
                  raise ValidationError(_('Check-in date should be greater than \
                                           the current date.'))
              if self.checkout < self.checkin:
                  raise ValidationError(_('Check-out date should be greater \
                                           than Check-in date.'))
docs: When date_order is less then check-in date or
          Checkout date should be greater than the check-in date.