bootstrap.php已经加载了orm、database

在classes/models也创建了topic.php。内容如下：

 
<?php
!defined('SYSPATH') && exit('Access denied');
 
class Model_Topic extends ORM {
 
    protected $primary_key = 'topic_id';
 
    protected $_belongs_to = array(
                'author'        => array(
                        'model'                 => 'member',
                        'foreign_key'   => 'topic_member_id',
                )
        );
}

classes/controller/home.php如下：

 
<?php
 
class Controller_Home extends Controller {
    private $obj_user;
    public function before() {
        $this->obj_user = ORM::Factory('topic');
    }
    public function action_index() {
 
        $this->request->resphome/index')->set('date',date('m/d/y'))->bind('test',$test);
 
        $test = 'hello';
    }
}
 

提示：ErrorException [ Fatal Error ]: Class 'Model_Topic' not found.
Why?