cssd-back/app/Http/Resources/TransferResource.php

<?php

namespace App\Http\Resources;

use Illuminate\Http\Request;
use Illuminate\Http\Resources\Json\JsonResource;

class TransferResource extends JsonResource
{
    /**
     * Transform the resource into an array.
     *
     * @param  Request  $request
     * @return array
     */
    public function toArray($request)
    {
        return [
            'id' => $this->id,
            'from_date' => $this->from_date,
            'to_date' => $this->to_date,
            'status' => $this->status,
            'user_id' => $this->user_id,
            'user' => UserResource::make($this->whenLoaded('user')),
            'transferable' => $this->resourceFinder(),
            'created_at' => $this->created_at,
            'updated_at' => $this->updated_at
        ];
    }

    private function resourceFinder()
    {
        if ($this->whenLoaded('transferable')) {
            if ($this->transferable_type == "App\\Package"){
                return PackageResource::make($this->transferable);
            }elseif($this->transferable_type == "App\\Item"){
                return ItemResource::make($this->transferable);
            }
        }
    }
}