public function addOrderBy($orderBy)
    {
        if ($orderBy == null)
            return;
        if ($this->_orderBy == null)
        {
            $this->_orderBy = $orderBy;
        }
        elseif (strpos($this->_orderBy, $orderBy) === false)
        {
            $this->_orderBy .= " AND " . $orderBy;
        }
    }