protected function getPropertyNames()
    {
        $propertyNames = array('id', 'pid', 'tstamp', 'sorting');

        if ($this->getItem()->getMetaModel()->hasVariants()) {
            $propertyNames[] = 'varbase';
            $propertyNames[] = 'vargroup';
        }

        return array_merge($propertyNames, array_keys($this->getItem()->getMetaModel()->getAttributes()));
    }