开发文档 需解释

二、数据修正：

$bug = fixer::input('post')
->add('openedBy', $this->app->user->account)
->add('openedDate', $now)
->setDefault('project,story,task', 0)
->setDefault('openedBuild', '')
->setIF($this->post->assignedTo != '', 'assignedDate', $now)
->setIF($this->post->story != false, 'storyVersion', $this->loadModel('story')->getVersion($this->post->story))
->specialChars('title,steps,keyword')
->cleanInt('product, module, severity')
->join('openedBuild', ',')
->remove('files, labels')

->get();

倒数第三行是什么意思?  文档上说是将 openedBuild 用','连接起来. ->setDefault('openedBuild', '')不是设了默认了吗? 为什么还要用,连接.实在看不懂.