Resource 为什么只能$this->user->id正常却不能this->comment->id

文章Models 已经定义了

 public function user()
    {
        return $this->belongsTo(User::class);
    }

    public function comment()
    {
        return $this->belongsTo(Comment::class);
    }

这个是文章的Resource

    public function toArray($request)
    {
//    dd($this->comment->id) 正常获取到ID
//    dd($this->user->id) 正常获取到ID
        return [
            $this->user->id, //正常获取到id
            $this->comment,//正常获取到模型中的所有字段信息
              $this->comment->id,//此处报错
//令我疑惑的是为什么同样是关联模型只有comment->id 报错  下面是报错信息
        ];
    }

{
    "message": "Attempt to read property \"id\" on null",
    "exception": "ErrorException",
    "file": "/Users/testApp/app/Http/Resources/Article.php",
    "line": 22,
    "trace": [
        {
            "file": "/Users/testApp/app/Http/Resources/Article.php",
            "line": 22,
            "function": "handleError",
            "class": "Illuminate\\Foundation\\Bootstrap\\HandleExceptions",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Collections/HigherOrderCollectionProxy.php",
            "line": 60,
            "function": "toArray",
            "class": "App\\Http\\Resources\\Article",
            "type": "->"
        },
        {
            "function": "Illuminate\\Support\\{closure}",
            "class": "Illuminate\\Support\\HigherOrderCollectionProxy",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Collections/Collection.php",
            "line": 695,
            "function": "array_map"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Collections/HigherOrderCollectionProxy.php",
            "line": 61,
            "function": "map",
            "class": "Illuminate\\Support\\Collection",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Http/Resources/Json/ResourceCollection.php",
            "line": 102,
            "function": "__call",
            "class": "Illuminate\\Support\\HigherOrderCollectionProxy",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Http/Resources/Json/JsonResource.php",
            "line": 95,
            "function": "toArray",
            "class": "Illuminate\\Http\\Resources\\Json\\ResourceCollection",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Http/Resources/Json/PaginatedResourceResponse.php",
            "line": 19,
            "function": "resolve",
            "class": "Illuminate\\Http\\Resources\\Json\\JsonResource",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Http/Resources/Json/ResourceCollection.php",
            "line": 134,
            "function": "toResponse",
            "class": "Illuminate\\Http\\Resources\\Json\\PaginatedResourceResponse",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Http/Resources/Json/ResourceCollection.php",
            "line": 114,
            "function": "preparePaginatedResponse",
            "class": "Illuminate\\Http\\Resources\\Json\\ResourceCollection",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Routing/Router.php",
            "line": 802,
            "function": "toResponse",
            "class": "Illuminate\\Http\\Resources\\Json\\ResourceCollection",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Routing/Router.php",
            "line": 789,
            "function": "toResponse",
            "class": "Illuminate\\Routing\\Router",
            "type": "::"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Routing/Router.php",
            "line": 721,
            "function": "prepareResponse",
            "class": "Illuminate\\Routing\\Router",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 128,
            "function": "Illuminate\\Routing\\{closure}",
            "class": "Illuminate\\Routing\\Router",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Routing/Middleware/SubstituteBindings.php",
            "line": 50,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "Illuminate\\Routing\\Middleware\\SubstituteBindings",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Routing/Middleware/ThrottleRequests.php",
            "line": 127,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Routing/Middleware/ThrottleRequests.php",
            "line": 103,
            "function": "handleRequest",
            "class": "Illuminate\\Routing\\Middleware\\ThrottleRequests",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Routing/Middleware/ThrottleRequests.php",
            "line": 55,
            "function": "handleRequestUsingNamedLimiter",
            "class": "Illuminate\\Routing\\Middleware\\ThrottleRequests",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "Illuminate\\Routing\\Middleware\\ThrottleRequests",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Auth/Middleware/Authenticate.php",
            "line": 44,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "Illuminate\\Auth\\Middleware\\Authenticate",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/sanctum/src/Http/Middleware/EnsureFrontendRequestsAreStateful.php",
            "line": 33,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 128,
            "function": "Laravel\\Sanctum\\Http\\Middleware\\{closure}",
            "class": "Laravel\\Sanctum\\Http\\Middleware\\EnsureFrontendRequestsAreStateful",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 103,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/sanctum/src/Http/Middleware/EnsureFrontendRequestsAreStateful.php",
            "line": 34,
            "function": "then",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "Laravel\\Sanctum\\Http\\Middleware\\EnsureFrontendRequestsAreStateful",
            "type": "->"
        },
        {
            "file": "/Users/testApp/app/Http/Middleware/AcceptHeader.php",
            "line": 20,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "App\\Http\\Middleware\\AcceptHeader",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 103,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Routing/Router.php",
            "line": 723,
            "function": "then",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Routing/Router.php",
            "line": 698,
            "function": "runRouteWithinStack",
            "class": "Illuminate\\Routing\\Router",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Routing/Router.php",
            "line": 662,
            "function": "runRoute",
            "class": "Illuminate\\Routing\\Router",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Routing/Router.php",
            "line": 651,
            "function": "dispatchToRoute",
            "class": "Illuminate\\Routing\\Router",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Foundation/Http/Kernel.php",
            "line": 167,
            "function": "dispatch",
            "class": "Illuminate\\Routing\\Router",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 128,
            "function": "Illuminate\\Foundation\\Http\\{closure}",
            "class": "Illuminate\\Foundation\\Http\\Kernel",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/livewire/livewire/src/DisableBrowserCache.php",
            "line": 19,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "Livewire\\DisableBrowserCache",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/nova/src/Http/Middleware/ServeNova.php",
            "line": 24,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "Laravel\\Nova\\Http\\Middleware\\ServeNova",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/beyondcode/laravel-query-detector/src/QueryDetectorMiddleware.php",
            "line": 33,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "BeyondCode\\QueryDetector\\QueryDetectorMiddleware",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/barryvdh/laravel-debugbar/src/Middleware/InjectDebugbar.php",
            "line": 67,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "Barryvdh\\Debugbar\\Middleware\\InjectDebugbar",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/TransformsRequest.php",
            "line": 21,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/ConvertEmptyStringsToNull.php",
            "line": 31,
            "function": "handle",
            "class": "Illuminate\\Foundation\\Http\\Middleware\\TransformsRequest",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "Illuminate\\Foundation\\Http\\Middleware\\ConvertEmptyStringsToNull",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/TransformsRequest.php",
            "line": 21,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/TrimStrings.php",
            "line": 40,
            "function": "handle",
            "class": "Illuminate\\Foundation\\Http\\Middleware\\TransformsRequest",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "Illuminate\\Foundation\\Http\\Middleware\\TrimStrings",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/ValidatePostSize.php",
            "line": 27,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "Illuminate\\Foundation\\Http\\Middleware\\ValidatePostSize",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/PreventRequestsDuringMaintenance.php",
            "line": 86,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "Illuminate\\Foundation\\Http\\Middleware\\PreventRequestsDuringMaintenance",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/fruitcake/laravel-cors/src/HandleCors.php",
            "line": 52,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "Fruitcake\\Cors\\HandleCors",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Http/Middleware/TrustProxies.php",
            "line": 39,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 167,
            "function": "handle",
            "class": "Illuminate\\Http\\Middleware\\TrustProxies",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 103,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Foundation/Http/Kernel.php",
            "line": 142,
            "function": "then",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/Users/testApp/vendor/laravel/framework/src/Illuminate/Foundation/Http/Kernel.php",
            "line": 111,
            "function": "sendRequestThroughRouter",
            "class": "Illuminate\\Foundation\\Http\\Kernel",
            "type": "->"
        },
        {
            "file": "/Users/testApp/public/index.php",
            "line": 52,
            "function": "handle",
            "class": "Illuminate\\Foundation\\Http\\Kernel",
            "type": "->"
        },
        {
            "file": "/Users/testApp/server.php",
            "line": 21,
            "function": "require_once"
        }
    ]
}