Tell SearchWP 3.x to index the Title from a Relationship ACF field

functions.php

<?php

/**
 * SearchWP VERSION 3.x
 * Tell SearchWP to index the Title from a Relationship ACF field.
 */
function my_searchwp_read_next_field( $meta_value, $meta_key, $the_post ) {
    $acf_field_name = 'read_next'; // The ACF Relationship field name.

    // If we're not indexing the Read Next field, return the existing meta value.
    // This logic also works for sub-fields of an ACF field as well.
    if ( $acf_field_name !== substr( $meta_key, strlen( $meta_key ) - strlen( $acf_field_name ) ) ) {
        return $meta_value;
    }

    // We're going to store all of our Titles together as one string for SearchWP to index.
    $titles = '';
    foreach ( (array) $meta_value as $entry ) {
        if ( is_numeric( $entry ) ) {
            // ACF stores only the post ID but we want the Title.
            $titles .= ' ' . get_the_title( absint( $entry ) );
        }
    }

    // $titles contains all of the Titles from the ACF Relationship field.
    return $titles;
}

add_filter( 'searchwp_custom_fields', 'my_searchwp_read_next_field', 99, 3 );