Test for https://github.com/contao/core/issues/6481

updateTest.php

<?php

/**
* Initialize the system
*/
define('TL_MODE', 'FE');
require 'system/initialize.php';

$tl_filesIDs = array('3', '4', '5', '6');

$DB = \Database::getInstance();

$DB->query('DROP TABLE IF EXISTS `update_test`');
$DB->query('
	CREATE TABLE `update_test` (
	  `id` int(9) unsigned auto_increment NOT NULL,
	  `orgid` int(9) unsigned NOT NULL,
	  `fld211` varchar(255) NOT NULL,
	  `fld30` int(9) unsigned NOT NULL,
	  PRIMARY KEY (`id`)
	) ENGINE=MyISAM DEFAULT CHARSET=utf8;
');

foreach($tl_filesIDs as $id) {
	$file = $DB->execute('SELECT * FROM tl_files WHERE id='.$id);
	if(!$file->numRows) die("tl_files.id=$id not found!\n");
	$DB->prepare('INSERT INTO update_test SET fld211=?, fld30=?, orgid=?')->execute($file->path, $file->id, $file->id);
}

for($i=0; $i<3; $i++)
{
	$u = new \Database\Updater();
	$u->convertSingleField('update_test', 'fld211');
	$u->convertSingleField('update_test', 'fld30');

	foreach($tl_filesIDs as $id)
	{
		$file = $DB->execute('SELECT * FROM tl_files WHERE id='.$id);
		$newrow = $DB->execute('SELECT * FROM update_test WHERE orgid='.$id);
		if($file->uuid != $newrow->fld211) die("Test FAILED for row {$newrow->id}\n");
		if($file->uuid != $newrow->fld30) die("Test FAILED for row {$newrow->id}\n");
	}
}

die("PASS\n");