private Node getPersonsRootNode( GraphDatabaseService graphDb ){
        Relationship rel = graphDb.getReferenceNode().getSingleRelationship( REF_PERSONS, Direction.OUTGOING );
        if ( rel != null ){
            return rel.getEndNode();
        }
        else{
            Transaction tx = this.graphDb.beginTx();
            try{
                Node refNode = this.graphDb.createNode();
                this.graphDb.getReferenceNode().createRelationshipTo( refNode, REF_PERSONS );
                tx.success();
                return refNode;
            }
            finally{
                tx.finish();
            }
        }
    }