protected function _mergePropertyData($current, $parent, $isAssoc)
    {
        if (!$isAssoc) {
            return array_merge($parent, $current);
        }
        $parent = Hash::normalize($parent);
        foreach ($parent as $key => $value) {
            if (!isset($current[$key])) {
                $current[$key] = $value;
            }
        }

        return $current;
    }