$order->items ()->make ([...]) 的疑问

下面这段代码是教材里的写法

    $sku = ProductSku::find($data['sku_id']);
    // 创建一个 OrderItem 并直接与当前订单关联
    $item = $order->items()->make([
        'amount' => $data['amount'],
        'price'  => $sku->price,
    ]);
    $item->product()->associate($sku->product_id);
    $item->productSku()->associate($sku);
    $item->save();

感觉不够简洁，于是我简化成如下这段

    $sku = ProductSku::find($data['sku_id']);
    $order->items()->make([
        'amount' => $data['amount'],
        'price'  => $sku->price,
        'product_id'=>$sku->product->id, //外键1赋值无效
        'product_sku_id'=>$sku->id,         //外键2赋值无效
    ])->save();

可为什么这样写就不行呢？程序报错：'product_id' doesn't have a default value
可我明明在数组里面给外键1和外键2都设过值了，为什么会无效呢？