A Laravel macro that adds a 'sync' method to the 'HasMany' relationship similar to the one on the 'BelongsTo' relationship.

AppServiceProvider.php

<?php

namespace App\Providers;

use Illuminate\Support\Arr;
use Illuminate\Support\ServiceProvider;
use Illuminate\Database\Eloquent\Builder;
use Illuminate\Database\Eloquent\Relations\HasMany;

class AppServiceProvider extends ServiceProvider
{
    public function register()
    {
    }


    public function boot()
    {
        HasMany::macro('sync', function (array $values = []) {
            $newValues = collect($values);

            $ids = collect($values)->pluck('id')->toArray();
            $oldIds = $this->getResults()->pluck('id')->toArray();

            $toDelete = array_diff($oldIds, $ids);
            if ($toDelete > 0) {
                $this->query->whereIn('id', $toDelete)->delete();
            }

            $relatedKey = explode('.', $this->foreignKey)[1];

            $newValues->each(function ($value) use ($relatedKey) {
                $finalValues = array_merge(Arr::except($value, ['id']), [$relatedKey => $this->parent->id]);
                $this->related->updateOrCreate(
                    [$relatedKey => $this->parent->id, 'id' => $value['id'] ?? 0],
                    $finalValues
                );
            });
        });
    }
}