Tell SearchWP 4 to index the Title from a Relationship ACF field instead of the post ID

searchwp-customizations.php

<?php

/**
 * Tell SearchWP to index the Title from a Relationship ACF field instead of the post ID
 */
add_filter( 'searchwp\source\post\attributes\meta', function( $meta_value, $args ) {
  $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( $args['meta_key'], strlen( $args['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.
  $content_to_index = '';
  if ( is_array( $meta_value ) && ! empty( $meta_value ) ) {
    foreach ( $meta_value[0] as $acf_relationship_item ) {
      if ( is_numeric( $acf_relationship_item ) ) {
        // ACF stores only the post ID but we want the Title.
        $content_to_index .= ' ' . get_the_title( absint( $acf_relationship_item ) );
      
        // If you want to index anything else, you can append it to $content_to_index.
      }
    }
  }

  // Return the string of content we want to index instead of the data stored by ACF.
  return $content_to_index;
}, 20, 2 );