protected function roundRobinGet($field)
    {
        if (empty($this->$field) || !is_array($this->$field)) {
            return '';
        }
        $val = $this->$field;
        $idx = $field . 'Index';
        if (!isset($this->$idx) || $this->$idx >= count($val)) {
            $this->$idx = 0;
        }
        return $val[ $this->$idx++ ];
    }