private function updateOID() {
    $pkValues = [];
    // collect the values of the primary keys and compose the oid from them
    $pkNames = $this->getMapper()->getPkNames();
    foreach ($pkNames as $pkName) {
      $pkValues[] = self::getValue($pkName);
    }
    $this->oid = new ObjectId($this->getType(), $pkValues);
  }