Codeigniter无法从模型中找到set_userdata（）

The Session library is autoloaded

My model:

class Login_model extends CI_Model{
    function __construct(){
        parent::__construct();
    }

    public function validate(){
        $username = $this->security->xss_clean($this->input->post('username'));
        $password = $this->security->xss_clean($this->input->post('password'));

        $this->db->where('username', $username);
        $this->db->where('password', $password);
        $query = $this->db->get('users');

        if($query->num_rows == 1)
        {
            $row = $query->row();
            $data = array(
                    'id' => $row->id,
                    'first_name' => $row->first_name,
                    'last_name' => $row->last_name,
                    'username' => $row->username,
                    'validated' => true
                    );
            $this->session->set_userdata($data);    // ### line 28 ### 
            return true;
        }
        return false;
    }
}

Gives this error:

Fatal error: Call to undefined method Session::set_userdata() in /var/www/codeIgniterTest/_application/models/login_model.php on line 28

Have you loaded the Session library?

class Login_model extends CI_Model {

    function __construct()
    {
        parent::__construct();
        $this->load->library('session');
    }

Try debugging and check all the included files by.

$included_files = get_included_files();

foreach ($included_files as $filename) {
    echo "$filename
";
}

I found the problem. I created my own Session class, but forgot to extend the CI_Session.

Thanks for the help!