Tree add child with empyty root give my root = 0, NestedTreeRepository search null

Question

i have entity

/**
 * @Gedmo\Tree(type="nested")
 * @ORM\Table(name="mKeyword")
 * use repository for handy tree functions
 * @ORM\Entity(repositoryClass="KeywordRepository")
 */
class Keyword {

/**
     * @Gedmo\TreeRoot
     * @ORM\Column(name="root", type="integer", nullable=true)
     */
    private $root;

when i add keyword

$node = new Keyword();

        $node->setTitle($data['nm']);

            $parent = $this->get_node($parentId);

            $node->setParent($parent);

        $em = $this->getDoctrine()->getManager();

        $em->persist($node);

i have in database root_id = 0

when i use

$this->db = $this->getDoctrine()->getRepository("MeaKeywordsBundle:Keyword")
$tree = $this->db->getChildren($node,true);

db is Gedmo\Tree\Entity\Repository\NestedTreeRepository

i get empty results. i see in query i have where root_id is null. But in database i have 0