Handling foreign key exceptions in PHP

Question

What is the best way in PHP to handle foreign key exceptions on a mysql database? Is there a mysql class that can be used to simplify any code?

Ideally, what I want

Answer 1

The way I handle this is to set up my database wrapper class to always throw an exception when you encounter a database error. So, for instance, I might have a class called MySQL with the following functions:

public function query($query_string)
{
    $this->queryId = mysql_query($query_string,$this->connectionId);
    if (! $this->queryId) {
        $this->_throwException($query_string);
    }
    return $this->queryId;
}

private function _throwException($query = null)
{
    $msg = mysql_error().".  Query was:\n\n".$query.
                "\n\nError number: ".mysql_errno();
    throw new Exception($msg,mysql_errno());
}

Any time a query fails, a regular PHP exception is thrown. Note that I would throw these from within other places too, like a connect() function or a selectDb() function, depending on whether the operation succeeded or not.

With that set up, you're good to go. Any place you expect that you might need to be handling a database error, do something like the following:

//assume $db has been set up to be an instance of the MySQL class

try {
    $db->query("DELETE FROM parent WHERE id=123");
} catch (Exception $e) {
    //uh-oh, maybe a foreign key restraint failed?
    if ($e->getCode() == 'mysql foreign key error code') {
        //yep, it failed.  Do some stuff.
    }
}

Edit

In response to the poster's comment below, you have some limited information available to you to help diagnose a foreign key issue. The error text created by a failed foreign key restraint and returned by mysql_error() looks something like this:

Cannot delete or update a parent row:
a foreign key constraint fails
(`dbname`.`childtable`, CONSTRAINT `FK_name_1` FOREIGN KEY
(`fieldName`) REFERENCES `parenttable` (`fieldName`));

If your foreign keys are complex enough that you can't be sure what might cause a foreign key error for a given query, then you could probably parse this error text to help figure it out. The command SHOW ENGINE INNODB STATUS returns a more detailed result for the latest foreign key error as well.

Otherwise, you're probably going to have to do some digging yourself. The following query will give you a list of foreign keys on a given table, which you can examine for information:

select * from information_schema.table_constraints
WHERE table_schema=schema() AND table_name='table_name';

Unfortunately, I don't think there's a magic bullet to your solution other than examining the errors and constraints very carefully.