遇到错误.无法加载请求的类:数据库

application/controller/目录中名为logincontroller.php的登录控制器是：

<?php
class LoginController extends CI_Controller {
public function __construct() {
    parent::__construct();
    $this->load->model('LoginModel');
    $this->load->library('form_validation');
    $this->load->database();
} 

public function index(){
$this ->load ->view('login_view');
}
public function checklogin(){
    $this->form_validation-> set_rules('email', 'email', 'required|valid_email');
    $this->form_validation-> set_rules('password', 'password', 'required|callback_verifyUser');
    if($this -> form_validation ->run() == false){
        $this->load->view('login_view');
    }else{
             redirect('dashcontroller/index');
    }
}

public function verifyUser(){
    $email = $this -> input -> post('email');
    $password= $this -> input ->post('password');
    $this->load->model ('LoginModel');
    if($this->LoginModel->login($email, $password)) {
        return true;
    }else{
        $this ->form_validation->set_message('verifyUser','wrong email or password');
        return false;
}
}

}？>

为什么会出现错误

我也在数据库的autoload.php中更改为$autoload['libraries'] = array('database');

我收到错误：无法加载请求的类：数据库