您需要首先加载数据库。 Codeiginiter将不会默认为您加载。

You need to load the database first. Codeiginiter won't load it by default for you.

您可以将其添加到 /config/autoload.php like like

You can either add it to /config/autoload.php like so

$autoload['libraries'] = array('database');

或者您可以通过调用

$this->load->database();

更多详情

http://codeigniter.com/user_guide/database/connecting.html