Tell SearchWP to index Posts from ACF Relationship Field

searchwp-customizations.php

<?php

// Customize ACF Relationship field data. By default an array of IDs is stored,
// this customization will instead tell SearchWP to index the full WP_Post object.
add_filter( 'searchwp\source\post\attributes\meta', function( $meta_value, $data ) {
  $acf_field_names = [ 
    'acf_relationship_field_name_1',
    'acf_relationship_field_name_2',
    'acf_relationship_field_name_3',
  ];
  
  if ( is_array( $data['meta_value'] ) && 1 === count( $data['meta_value'][0] ) ) {
    $data['meta_value'] = $data['meta_value'][0];
  }
  
  foreach ( $acf_field_names as $acf_field_name ) {
    if ( $acf_field_name == $data['meta_key'] ){
      $meta_value = array_map( function( $post_id ) {
        return is_numeric( $post_id ) ? get_post( $post_id ) : $post_id;
      }, $data['meta_value'] );

      break;
    }
  }

  return $meta_value;
}, 10, 2 );