Pimcore\Model\AbstractModel::determineResourceClass PHP Method

    protected function determineResourceClass($className)
    {
        $filesToInclude = [];
        $filePath = str_replace(["_", "\\"], "/", $className) . ".php";
        $filesToInclude[] = preg_replace("@^Pimcore/Model/@", "", $filePath);
        $filesToInclude[] = $filePath;
        foreach ($filesToInclude as $fileToInclude) {
            if ($fileToInclude == "Dao.php" || $fileToInclude == "Resource.php") {
                return;
            }
            if (File::isIncludeable($fileToInclude)) {
                include_once $fileToInclude;
                if (Tool::classExists($className)) {
                    return $className;
                }
            }
        }
        return;
    }

 /**
  * determineResourceClass
  *
  * @param $className
  */
 protected function determineResourceClass($className)
 {
     if (Tool::classExists($className)) {
         return $className;
     }
     return parent::determineResourceClass($className);
 }