Cannot delete or update a parent row: a foreign key constraint fails. Foreign keys exist though

Question

Why do I get this when I want to delete a row in webapp_requests?

Cannot delete or update a parent row: a foreign key constraint fails (`databasename`.`webapp_requestsuser`, CONSTRAINT `fk_perRequests` FOREIGN KEY (`requestsid`) REFERENCES `webapp_requests` (`id`))

CREATE TABLE `webapp_requests` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`userid` int(11) NOT NULL,
`opentime` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
`starttime` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
`description` longtext,
`duration` double DEFAULT '0',
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8

CREATE TABLE `webapp_requestsuser` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user` int(11) NOT NULL,
`status` tinyint(4) DEFAULT '0',
`requestsid` int(11) NOT NULL,
PRIMARY KEY (`id`),
KEY `requestsid` (`requestsid`),
CONSTRAINT `fk_perRequests` FOREIGN KEY (`requestsid`) REFERENCES `webapp_requests` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8

Content webapp_requests:

_______________________________________
| id          = 2                     |
| userid      = 2                     |
| opentime    = 2015-11-24 16:26:44   |
| starttime   = 2015-11-24 16:26:44   |
| description = This is a description |
| duration    = 5.0                   |
_______________________________________

Content webapp_requestsuser:

_______________________________________
| id          = 3                     |
| user        = 1                     |
| status      = 0                     |
| requestsid  = 2                     |
_______________________________________

DELETE FROM `webapp_requests` WHERE `id` = 2;

I use this to Insert data:

public function createRequest($userid, $recipients, $description, $duration, $starttime)
{
    $existcheck = $this->db->query('SELECT * FROM `webapp_requests` WHERE `userid`=('.$this->db->escape($userid).')');
    // Check if already a request is registered
    if ($existcheck->num_rows() > 0) {
        return false;
    }
    // First add the request
    $query = $this->db->query('INSERT INTO `webapp_requests`( `userid`, `description`, `duration`, `starttime`) VALUES (' . $this->db->escape($userid) . ',' . $this->db->escape($description) . ',' . $this->db->escape($duration) . ',' . $this->db->escape($starttime) . ')');
    $id = $this->db->insert_id();
    if (!$query) {
        return false;
    } else {
        // Then add the users to the request
        $tokens = [];
        foreach($recipients as $recipient) {
            $this->db->query('INSERT INTO `webapp_requestsuser`(`user`,`requestsid`) VALUES (' . $this->db->escape($recipient) . ',' . $this->db->escape($id) . ')');
            $pushValue = $this->db->query('SELECT * FROM `notificationtoken` WHERE `user`=('.$this->db->escape($recipient).')')->row();
            array_push($tokens, $pushValue);
        }
        return $tokens;
    }
}

Answer 1

Your "creates" work fine.

The problem is that you are deleting a row in the referenced table (webapp_requests) that would violate the constraint.

So it would orphan a row in the referencing table (webapp_requestsuser). There would be at least one row there that says where is my parent now? My requestsid is a 2, but there is not an id=2 in referenced table anymore.

So the dbengine says, you told me not to allow it, and I am not allowing it.

Cascading deletes

For those that want to see a cascading delete example, see an answer of mine here.