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.'))
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