Question

I have a table called mail with a foreign key column called deletedBy which references the users table by pk. when I create a new mail i want to set this field to null as the mail is undeleted, however when i do i get an foreign key constraint failure error.

I totally understand the error, but how do i get around it?

table:-

CREATE TABLE `mail` (
  `pk` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `from` int(11) unsigned NOT NULL,
  `to` int(11) unsigned NOT NULL,
  `subject` varchar(255) NOT NULL,
  `message` text NOT NULL,
  `dt` datetime NOT NULL,
  `read` enum('0','1') NOT NULL DEFAULT '0',
  `deletedby` int(11) unsigned DEFAULT '0',
  PRIMARY KEY (`pk`),
  KEY `from_mail__users_pk` (`from`),
  KEY `to_mail__users_pk` (`to`),
  KEY `deletedby_mail__users_pk` (`deletedby`),
  CONSTRAINT `deletedby_mail__users_pk` FOREIGN KEY (`deletedby`) REFERENCES `user` (`pk`) ON DELETE NO ACTION ON UPDATE NO ACTION,
  CONSTRAINT `from_mail__users_pk` FOREIGN KEY (`from`) REFERENCES `user` (`pk`) ON DELETE NO ACTION ON UPDATE NO ACTION,
  CONSTRAINT `to_mail__users_pk` FOREIGN KEY (`to`) REFERENCES `user` (`pk`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8 COLLATE=utf8_bin

error message:-

 ERROR 1452: Cannot add or update a child row: a foreign key constraint fails (`ajfit`.`mail`, CONSTRAINT `deletedby_mail__users_pk` FOREIGN KEY (`deletedby`) REFERENCES `user` (`pk`) ON DELETE NO ACTION ON UPDATE NO ACTION)
SQL Statement:
 INSERT INTO `ajfit`.`mail` (`from`, `to`, `subject`, `message`, `read`) VALUES (31, 30, 'test', 'test', '0')

Thanks

Andrew

Answer 1

Because you are not providing a value for deletedby, the DEFAULT value of 0 used. But there is (presumably) no record in the user table with a pk of 0 and therefore the foreign key constraint (correctly) fails.

As you have said yourself, you want deletedby to have a DEFAULT value of NULL so that the foreign key constraint is not checked until an alternative value is provided:

ALTER TABLE mail MODIFY deletedby int(11) unsigned DEFAULT NULL;