xml mapping : embedded-document node is ignored, can't persist

 
//file Doctrine/ODM/MongoDB/Mapping/Driver/XmlDriver.php line 171

protected function _loadMappingFile($file)
    {
        $result = array();
        $xmlElement = simplexml_load_file($file);

        if (isset($xmlElement->document)) {
            foreach ($xmlElement->document as $documentElement) {
                $documentName = (string) $documentElement['name'];
                $result[$documentName] = $documentElement;
            }
        }
        
        return $result;
    }