exchange-2.0/app/Http/Resources/MilestoneResource.php

<?php

namespace App\Http\Resources;

use Illuminate\Http\Resources\Json\JsonResource;

class MilestoneResource extends JsonResource
{
    /**
     * Transform the resource into an array.
     *
     * @param  \Illuminate\Http\Request $request
     * @return array
     */
    public function toArray($request)
    {
        $rewardIds = $this->rewards->pluck('id')->map(function ($id) {
            return (int) $id;
        })->toArray();

        return [
            'id' => $this->id,
            'name' => $this->name,
            'description' => $this->description,
            'reward_ids' =>  $rewardIds
        ];
    }
}