Laravel 5.2 Eloquent模型创建于现在

我在Laravel 5.2中遇到一个Eloquent模型问题。我的模型是User，Authenticatable的子类，每当我将名为last_activity的日期字段设置为Carbon::now()时，它也会将created_at设置为当前时间。

我正在使用此查询修改条目：

$user->last_activity = Carbon::now();
$user->save();

这是我的app'User.php：

namespace App;
use Illuminate'Foundation'Auth'User as Authenticatable;
use Illuminate'Database'Eloquent'SoftDeletes;
use Illuminate'Auth'Passwords'CanResetPassword;
class User extends Authenticatable
{
    use SoftDeletes, CanResetPassword;
    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = [
        'handle', 'email', 'password', 'confirmation_code'
    ];
    /**
     * The attributes excluded from the model's JSON form.
     *
     * @var array
     */
    protected $hidden = [
        'password', 'last_ip', 'updated_at', 'email', 'confirmed', 'confirmation_code', 'updated_at', 'deleted_at'
    ];
    protected $dates = ['last_logged_in', 'last_activity', 'created_at', 'updated_at', 'deleted_at'];
}