如何在Laravel Framework中使用MongoDB

我在将MongoDB与Laravel框架一起使用时遇到问题。我用了这个Laravel-MongoDB

这是我得到的错误

/app/model/User.php

<?php 

use Jenssegers\Mongodb\Model as Eloquent;

class User extends Eloquent {

     //protected $connection = 'mongodb';
     protected $collection = 'users';

     $user = User::all();

     public function all()
     {
        return $this->$user;
     }
}


?>

/app/routes.php

Route::get('users', function()
{

    $users = User::all();   
    return View::make('users')->with('users',$users);

});

/app/config/database.php

'mongodb' => array(
            'driver'   => 'mongodb',
            'host'     => 'localhost',
            'port'     => 27017,
            'username' => 'username',
            'password' => 'password',
            'database' => 'users'
        ),

我不知道我的实现有什么问题。请帮帮我..