表的结构如下：

REATE TABLE `carts` (
  `user_id` int(10) unsigned NOT NULL,
  `product_id` char(64) NOT NULL,
  `quantity` int(10) unsigned NOT NULL,
  `created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  PRIMARY KEY (`user_id`,`product_id`),
  KEY `fk_idx_cart_product` (`product_id`),
  CONSTRAINT `fk_idx_cart_product` FOREIGN KEY (`product_id`) REFERENCES `products` (`id`),
  CONSTRAINT `fk_idx_cart_user` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

需要用Eloquent ORM的save()方法，他会根据primaryKey来保存数据，请问如何在Model设置这种复合主键？

/**
 * The primary key for the model.
 *
 * @var string
 */
protected $primaryKey = 'id';

$cart = Cart::firstOrNew([...]);
$cart->quantity = $quantity;
$cart->save();

SQLSTATE[42S22]: Column not found: 1054 Unknown column 'id' in 'where clause'