Prevent to loaded One to One relationship on "Laravel API Resource"

Hello guys,

I have tried on docs laravel it's called "whenLoaded" on Laravel API Resource, the problem here is whenLoaded is work on Many relationship meanwhile one relationship must be called, so how can i prevent one to one is loaded on API Resource, considere this bellow code

Travel has one to Schedule

and Schedule one to Travel

and my resource like this

<?php

namespace App\Http\Resources;

use Illuminate\Http\Resources\Json\JsonResource;

class TravelResource extends JsonResource
{
    /**
     * Transform the resource into an array.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return array
     */
    public function toArray($request)
    {
        return [
            'schedule' => new TravelScheduleResource($this->schedule),
        ];
    }
}

<?php

namespace App\Http\Resources;

use Illuminate\Http\Resources\Json\JsonResource;

class TravelScheduleResource extends JsonResource
{
    /**
     * Transform the resource into an array.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return array
     */
    public function toArray($request)
    {
        return [
            'travel' => new TravelResource($this->travel),
        ];
    }
}

and my controller returned

return new ScheduleResource($resource);

and the result get an error

Maximum stack depth exceeded

Thanks in advance for reading my problem :)