sakura/app/Forum/ForumPerms.php

<?php
/**
 * Holds the forum permission handler.
 * @package Sakura
 */

namespace Sakura\Forum;

use Sakura\DB;
use Sakura\User;
use Sakura\PermissionHandler;
use Traversable;

/**
 * Forum permission handler.
 * @package Sakura
 * @author Julian van de Groep <me@flash.moe>
 */
class ForumPerms extends PermissionHandler
{
    public const TABLE = 'forum_perms';

    public function __construct(Forum $forum, User $user)
    {
        $this->additionalRequirements['where_in']['forum_id'] = iterator_to_array($this->getForumIds($forum->id));
        parent::__construct($user);
    }

    public function getForumIds(int $id): Traversable
    {
        // yield the initial id
        yield $id;

        while ($id > 0) {
            $id = DB::table('forums')
                ->where('forum_id', $id)
                ->value('forum_category');

            yield $id;
        }
    }
}