Add links to tournaments on events

sp_get_event_tournaments_with_example.php

<?php

function sp_get_event_tournaments($event_id)
{
	return get_posts([
		'post_type'      => 'sp_tournament',
		'posts_per_page' => -1,
		'meta_query'     => [
			[
				'key' => 'sp_event',
				'value' => $event_id,
			]
		]
	]);
}

/**
 * Add a player list on the fly to the current team.
 */
add_action(
	'init',
	function () {

		add_filter(
			'the_content',
			function ($content) {

				$additional_content = '';
				$object = get_queried_object();

				/**
				 * Exit if this is not a SportsPress event.
				 */
				if ('sp_event' !== $object->post_type) {
					return $content;
				}

				$tournaments = sp_get_event_tournaments($object->ID);

				if ($tournaments) {
					ob_start();
					?>
					<h3>Tournaments</h3>
					<ul>
					<?php foreach ($tournaments as $tournament) : ?>
						<li><a href="<?php the_permalink(); ?>"><?php echo get_the_title($tournament); ?></a></li>
					<?php endforeach; ?>
					</ul>
					<?php
					$additional_content = ob_get_clean();
				}

				/**
				 * Done
				 */
				return $content . $additional_content;
			},
			10
		);
	}
);