RSForm Helper to get a list of titles from published categories and subdirectories

_rsform-dropdown-items.field

//<code>
	require_once JPATH_SITE . '/templates/yourtemplate/html/com_rsform/custom/contact.php';
	return (new ContactHelper)->getCourses(1402);
//</code>

contact.php

<?php
/**
 * @package     Template
 * @copyright   2019 Perfect Web Team / perfectwebteam.nl
 * @license     GNU General Public License version 3 or later
 */

// stored in templates/yourtemplate/html/rsform/custom/contact.php

use Joomla\CMS\Factory;
use Joomla\CMS\Language\Text;
use Joomla\CMS\MVC\Model\BaseDatabaseModel;
use Joomla\Registry\Registry;

defined('_JEXEC') or die();

/**
 * Helper for contact form.
 *
 * @package     Template
 * @since       1.0.0
 */
class ContactHelper
{
	/**
	 * Get a list of courses to show.
	 *
	 * @param   integer  $categoryId  The parent category ID to get the articles for
	 *
	 * @return  string  The courses to show.
	 *
	 * @since   1.0.0
	 */
	public function getCourses(int $categoryId)
	{
		BaseDatabaseModel::addIncludePath(JPATH_ROOT . '/components/com_content/models/');

		/** @var ContentModelArticles $oModel */
		$oModel = BaseDatabaseModel::getInstance('Articles', 'ContentModel', array('ignore_request' => true));

		// Get the subcategories
		$catid = $this->getSubcategories($categoryId);
		$oModel->setState('filter.category_id', $catid);
		$oModel->setState('params', new Registry);
		$oModel->setState('filter.language', true);

		// We assume that any training that is no longer published, is finished so can be ignored
		$oModel->setState('filter.published', 1);
		$oModel->setState('filter.get_children', 5);
		$oModel->setState('list.ordering', 'c.rgt');

		$items = $oModel->getItems();

		$values = array();
		$values[] = '|' . Text::_('JSELECT');
		$group = '';

		foreach ($items as $item)
		{
			if ($group !== $item->category_title)
			{
				$group    = $item->category_title;
				$values[] = $group . '[g]';
			}

			$values[] = $item->title;
		}

		return implode("\n", $values);
	}

	/**
	 * Get a list of category IDs.
	 *
	 * @param   int $categoryId The category to get the sub IDs for
	 *
	 * @return  array  List of subcategory IDs.
	 *
	 * @since   1.0.0
	 */
	private function getSubcategories($categoryId)
	{
		// Get a list of content categories
		$db = Factory::getDbo();

		$query = $db->getQuery(true)
			->select($db->quoteName('node.id'))
			->from($db->quoteName('#__categories', 'node'))
			->from($db->quoteName('#__categories', 'parent'))
			->where($db->quoteName('node.lft') . ' BETWEEN ' . $db->quoteName('parent.lft') . ' AND ' . $db->quoteName('parent.rgt'))
			->where($db->quoteName('parent.id') . ' = ' . (int) $categoryId);
		$db->setQuery($query);

		return $db->loadColumn();
	}
}