public function getSubtree($post_id, $parent_id = null)
 {
     $post_id = (int) $post_id;
     $parent_id = (int) $parent_id;
     $q = new waDbQuery($this);
     if ($post_id) {
         $q->where("post_id = {$post_id}");
     }
     if ($parent_id) {
         $parent = $this->getById($parent_id);
         if (!$parent) {
             $q->where('id = ' . $parent_id);
         } else {
             $where = "`{$this->left}`  >= {$parent[$this->left]} AND\n                    `{$this->right}` <= {$parent[$this->right]}";
             $q->where($where);
         }
     }
     return $q->fetchAll('id');
 }