我有两个模型：Invoice和InvoiceItems，这些型号与invoice_id连接：

Schema::create('invoice_items', function (Blueprint $table) {
            $table->bigInteger('invoice_id')->unsigned();
            $table->foreign("invoice_id")->references('id')->on('invoices')->onDelete('cascade');
});

Invoice：

class Invoice extends Model
{
    protected $fillable = [
        'profile_id',
        'invoice_number',
         ....
    ];

    public function invoiceItems()
    {
        return $this->hasMany('App\InvoiceItem');
    }
 }

和InvoiceItems

class InvoiceItem extends Model
{
    protected $guarded = [];

    public function invoice()
    {
        return $this->belongsTo('App\Invoice');
    } 
}

当我想用它的发票项目使用雄辩的发票，我使用以下代码：

$invoice = Invoice::with('invoiceItems')->findOrFail($id);

现在dd($invoice->toArray())的输出是：

array:20 [▼
  "id" => 14
  "profile_id" => 13
  "invoice_number" => null
  ....
  "created_at" => "2020-05-05 09:21:46"
  "updated_at" => "2020-05-09 12:09:14"
  "invoice_items" => array:1 [▼
    0 => array:10 [▼
      "id" => 9
      "invoice_id" => 14
      "price" => "10000.00"
      "created_at" => "2020-05-05 09:21:46"
      "updated_at" => "2020-05-05 09:21:46"
    ]
  ]
]

到现在为止一切看起来都很好，但我很困惑！当我想使用$invoice->invoice_items时，它返回null，而它存在于顶部的dd($invoice->toArray())！另一方面，当我使用$invoice->invoiceItems时，它工作得很好！那么，造成这种差异的原因是什么呢？