koel/app/Rules/AllPlayablesAreAccessibleBy.php

<?php

namespace App\Rules;

use App\Models\User;
use App\Repositories\SongRepository;
use Closure;
use Illuminate\Contracts\Validation\ValidationRule;
use Illuminate\Support\Arr;

class AllPlayablesAreAccessibleBy implements ValidationRule
{
    public function __construct(private readonly User $user)
    {
    }

    /** @param array<string> $value */
    public function validate(string $attribute, mixed $value, Closure $fail): void
    {
        $ids = array_unique(Arr::wrap($value));

        if ($ids && app(SongRepository::class)->getMany(ids: $ids, scopedUser: $this->user)->count() !== count($ids)) {
            $fail('Not all songs or episodes exist in the database or are accessible by the user.');
        }
    }
}