public function update($ids)
    {
        if (empty($ids)) {
            return false;
        } elseif (empty($this->_ids)) {
            $this->_ids = $ids;
            $change = true;
        } else {
            $change = false;
            foreach ($ids as $k => $v) {
                if (!isset($this->_ids[$k]) || ($this->_ids[$k] != $v)) {
                    $this->_ids[$k] = $v;
                    $change = true;
                }
            }
        }

        if ($change) {
            $this->_sorted = false;
        }

        return $change;
    }