protected function _setupPrimaryAssignment()
    {
        if ($this->_primaryAssignment === null) {
            $this->_primaryAssignment = array(1 => self::PRIMARY_ASSIGNMENT_SESSION_ID);
        } else if (!is_array($this->_primaryAssignment)) {
            $this->_primaryAssignment = array(1 => (string) $this->_primaryAssignment);
        } else if (isset($this->_primaryAssignment[0])) {
            array_unshift($this->_primaryAssignment, null);

            unset($this->_primaryAssignment[0]);
        }

        if (count($this->_primaryAssignment) !== count($this->_primary)) {
            /**
             * @see Zend_Session_SaveHandler_Exception
             */
            require_once 'Zend/Session/SaveHandler/Exception.php';

            throw new Zend_Session_SaveHandler_Exception(
                "Value for configuration option '" . self::PRIMARY_ASSIGNMENT . "' must have an assignment "
              . "for each session table primary key.");
        } else if (!in_array(self::PRIMARY_ASSIGNMENT_SESSION_ID, $this->_primaryAssignment)) {
            /**
             * @see Zend_Session_SaveHandler_Exception
             */
            require_once 'Zend/Session/SaveHandler/Exception.php';

            throw new Zend_Session_SaveHandler_Exception(
                "Value for configuration option '" . self::PRIMARY_ASSIGNMENT . "' must have an assignment "
              . "for the session id ('" . self::PRIMARY_ASSIGNMENT_SESSION_ID . "').");
        }
    }