Tell SearchWP to incercept ACF Relationship Field searches

functions.php

<?php

// Callback to have SearchWP intercept ACF Relationship field searches.
function my_searchwp_acf_relationship_field_search( $args, $field, $post_id ) {
	if ( empty( $args['s'] ) || ! class_exists( 'SWP_Query' ) ) {
		return $args;
	}

	// Assume that the SearchWP engine to use is the defined admin search engine
	// from SearchWP's Advanced settings screen.
	$searchwp_advanced_settings = searchwp_get_option( 'advanced' );
	$searchwp_admin_engine = ! empty( $searchwp_advanced_settings['admin_engine'] ) 
		? $searchwp_advanced_settings['admin_engine'] : 'default';

	$searchwp_args = array(
		'engine' => $searchwp_admin_engine, // The SearchWP engine to use.
		's'      => $args['s'],             // Pass along the search query.
		'fields' => 'ids',                  // Return only post IDs.
	);

	if ( ! empty( $args['taxonomy' ] ) ) {
		$tax_arg = explode( ':', $args['taxonomy'] );

		$searchwp_args['tax_query'] = array(
			array(
				'taxonomy' => $tax_arg[0],
				'field'    => 'slug',
				'terms'    => $tax_arg[1],
			),
		);
	}

	if ( ! empty( $args['post_type'] ) ) {
		$searchwp_args['post_type'] = $args['post_type'];
	}

	// Tell SearchWP to NOT log this search.
	add_filter( 'searchwp_log_search', '__return_false' );

	// Retrieve SearchWP results.
	$results = new SWP_Query( $searchwp_args );

	// If there are no results, we need to force ACF to reflect that.
	if ( empty( $results->posts ) ) {
		$results->posts = array( 0 );
	}

	// We're going to use SearchWP's results to handle the restrictions as outlined.
	$args['s']        = '';
	$args['order']    = '';
	$args['orderby']  = 'post__in';
	$args['post__in'] = $results->posts;

	return $args;
}

// Tell SearchWP to intercept all ACF Relationship field searches.
add_filter(
	'acf/fields/relationship/query',
	'my_searchwp_acf_relationship_field_search',
	90,
	3
);

// Tell SearchWP to intercept a single ACF Relationship field search.
// add_filter(
// 	'acf/fields/relationship/query/name=my_acf_relationship_field_name',
// 	'my_searchwp_acf_relationship_field_search',
// 	100,
// 	3
// );