включить связанные ресурсы в ответ Resources

Есть 2 таблицы:

PHP:

1. <?php
2.  
3. use Illuminate\Support\Facades\Schema;
4. use Illuminate\Database\Schema\Blueprint;
5. use Illuminate\Database\Migrations\Migration;
6.  
7. class CreateSlidestructsTable extends Migration
8. {
9.     /**
10.      * Run the migrations.
11.      *
12.      * @return void
13.      */
14.     public function up()
15.     {
16.         Schema::create('slidestructs', function (Blueprint $table) {
17.             $table->increments('id');
18.             $table->unsignedInteger('number');
19.             $table->string('title', 2000);
20.             $table->integer('title_sign');
21.             $table->string('category', 2000);
22.             $table->string('slidestructs_image_url', 2000);
23.             $table->string('slidestructs_thumb_url', 2000);
24.             $table->integer('thumb_position')->nullable();
25.             $table->string('type', 100);
26.             $table->timestamps();
27.         });
28.     }
29.  
30.     /**
31.      * Reverse the migrations.
32.      *
33.      * @return void
34.      */
35.     public function down()
36.     {
37.         Schema::dropIfExists('slidestructs');
38.     }
39.  
40. }

PHP:

1. <?php
2.  
3. use Illuminate\Support\Facades\Schema;
4. use Illuminate\Database\Schema\Blueprint;
5. use Illuminate\Database\Migrations\Migration;
6.  
7. class CreateSlideimagesTable extends Migration
8. {
9.     /**
10.      * Run the migrations.
11.      *
12.      * @return void
13.      */
14.     public function up()
15.     {
16.         Schema::create('slideimages', function (Blueprint $table) {
17.             $table->increments('id');
18.             $table->unsignedInteger('slide_id');
19.             $table->foreign('slide_id')->on('slidestructs')->references('id');
20.             $table->string('slideimages_image_url', 2000);
21.             $table->timestamps();
22.         });
23.     }
24.  
25.     /**
26.      * Reverse the migrations.
27.      *
28.      * @return void
29.      */
30.     public function down()
31.     {
32.         Schema::dropIfExists('slideimages');
33.     }
34. }

Контроллер:

PHP:

1. <?php
2.  
3. namespace App\Http\Controllers;
4.  
5. use App\Http\Resources\SlideResource;
6. use App\Slidestruct;
7. use Illuminate\Http\Request;
8.  
9. class PresentationController extends Controller
10. {
11.     /**
12.      * Display a listing of the resource.
13.      *
14.      * @return \Illuminate\Http\Response
15.      */
16.     public function index()
17.     {
18.         $Struct = Slidestruct::with('slideimage')->get();
19.         return SlideResource::collection($Struct);
20.     }

И в api.php:

PHP:

1. Route::get('/', '\App\Http\Controllers\PresentationController@index');

Файл slide.php:

PHP:

1. <?php
2.  
3. namespace App\Http\Resources;
4.  
5. use Illuminate\Http\Resources\Json\JsonResource;
6.  
7. class Slide extends JsonResource
8. {
9.     /**
10.      * Transform the resource into an array.
11.      *
12.      * @param  \Illuminate\Http\Request  $request
13.      * @return array
14.      */
15.     public function toArray($request)
16.     {
17.         //return parent::toArray($request);
18.     }
19. }

Файл SlideResource.php:

PHP:

1. <?php
2.  
3. namespace App\Http\Resources;
4.  
5. use App\Slidestruct;
6. use Illuminate\Http\Resources\Json\JsonResource;
7. use Illuminate\Http\Resources\Json\ResourceCollection;
8.  
9. class SlideResource extends JsonResource
10. {
11.     /**
12.      * Transform the resource into an array.
13.      *
14.      * @param  \Illuminate\Http\Request  $request
15.      * @return array
16.      */
17.  
18.     public function toArray($request)
19.     {
20.         return [
21.           'id' => $this->id,
22.           'number' => $this->number,
23.           'title' => $this->title,
24.           'title_sign' => $this->title_sign,
25.           'category' => $this->category,
26.           'slidestructs_image_url' => $this->slidestructs_image_url,
27.           'slidestructs_image_thumb' => $this->slidestructs_image_url,
28.           'thumb_position' => $this->thumb_position,
29.           'type' => $this->type,
30.           'created_at' => $this->created_at,
31.           'updated_at' => $this->updated_at,
32.  
33.           'slideimages_image_url' => Slide::collection($this->slideimages_image_url),
34.         ];
35.     }
36. }

выдает ошибку: "Call to a member function first() on null",

хочу вытащить связанные данные с помощью кода:

PHP:

1. 'slideimages_image_url' => Slide::collection($this->slideimages_image_url),