Question

I have a problem with mutual constraints.
I want to have two tables each having a constraint on the other one.

I’m working with Doctrine2 (but it’s not related to the problem), here is my simplified code:

SQL:

CREATE TABLE IF NOT EXISTS `thread` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `last_message_id` int(11) DEFAULT NULL,
  `subject` varchar(255) NOT NULL
  PRIMARY KEY (`id`),
  UNIQUE KEY `UNIQ_C023F2BBBA0E79C3` (`last_message_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8;

ALTER TABLE `thread`
ADD CONSTRAINT `FK_C023F2BBBA0E79C3` FOREIGN KEY (`last_message_id`) REFERENCES `message` (`id`);

CREATE TABLE IF NOT EXISTS `message` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `user_id` int(11) DEFAULT NULL,
  `thread_id` int(11) DEFAULT NULL,
  `body` longtext NOT NULL
  PRIMARY KEY (`id`),
  KEY `IDX_9E4E8B5FA76ED395` (`user_id`),
  KEY `IDX_9E4E8B5FE2904019` (`thread_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8;

ALTER TABLE `message`
ADD CONSTRAINT `FK_9E4E8B5FE2904019` FOREIGN KEY (`thread_id`) REFERENCES `thread` (`id`) ON DELETE CASCADE;

Doctrine2 mapping (which generated the SQL code above):

<?php

class Thread
{
    /* @ORM\OneToOne() */
    private $lastMessage;
}

class Message
{
    /* @ORM\ManyToOne() */
    private $thread;
}

And when I try to delete either a thread or a message, I get (logically) the error:
Integrity constraint violation: 1451 Cannot delete or update a parent row: a foreign key constraint fails ('thread', CONSTRAINT 'FK_C023F2BBBA0E79C3' FOREIGN KEY ('last_message_id') REFERENCES 'message' ('id'))

So, is there a way to avoid this error?
Or should I forget mutual constraints?
Anything?

I want to add that I want to keep the last_message_id because I want to display the threads with infos on their last message, and making a (paginated) query without this reference to the last message was a total nightmare…

Thanks!

Answer 1

The best solution I can think of would be to add a ON DELETE CASCADE constraint to the FK on the Thread table. That way if you delete the thread, the associated messages would be automatically deleted as well.

Similarly, you would need to add a ON DELETE SET NULL constraint on the Messages table FK so that if you deleted the last message in a Thread, it would set the last_message_id to NULL on the Thread table.

Or you could just do logical (soft) deletes instead of hard deletes, which would also solve the problem.

ETA:

Now that you’ve posted the constraints, this is the one you would have to modify:

ALTER TABLE `thread`
ADD CONSTRAINT `FK_C023F2BBBA0E79C3` FOREIGN KEY (`last_message_id`) 
REFERENCES `message` (`id`) ON DELETE SET NULL;