fix error

lib/Gedmo/Tree/Strategy/ORM/Closure.php

@@ -201,8 +201,8 @@ class Closure implements Strategy
             $closureMeta = $em->getClassMetadata($closureClass);
             $closureTable = $closureMeta->getTableName();
 
-            $ancestorColumnName = $this->_getJoinColumnFieldName($em->getClassMetadata($config['closure'])->getAssociationMapping('ancestor'));
-            $descendantColumnName = $this->_getJoinColumnFieldName($em->getClassMetadata($config['closure'])->getAssociationMapping('descendant'));
+            $ancestorColumnName = $this->getJoinColumnFieldName($em->getClassMetadata($config['closure'])->getAssociationMapping('ancestor'));
+            $descendantColumnName = $this->getJoinColumnFieldName($em->getClassMetadata($config['closure'])->getAssociationMapping('descendant'));
             $depthColumnName = $em->getClassMetadata($config['closure'])->getColumnName('depth');
 
             $entries = array(