Flexible content relationship fields

relationship_fields.php

<?php

add_filter( 'relevanssi_content_to_index', 'rlv_relationship_content', 10, 2 );
function rlv_relationship_content( $content, $post ) {
	global $wpdb;
	$relationships = unserialize( $wpdb->get_var( "SELECT meta_value FROM $wpdb->postmeta WHERE meta_key LIKE 'sidebar_groups_%_people'" ) ); // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions
	if ( ! is_array( $relationships ) ) {
		$relationships = array( $relationships );
	}
	foreach ( $relationships as $related_post_id ) {
		$content .= ' ' . get_the_title( $related_post_id );
	}
	return $content;
}