Add custom variation data to WooCommerce products in the SearchWP index. More details at https://mircian.com/2017/09/20/woocommerce-variation-data-searchwp/

search_wp_index_woocommerce_variation_data.php

<?php

/**
 * @param array $extra_meta
 * @param WP_Post $post_being_indexed
 *
 * @return array
 */
function m_index_woocommerce_variation_isbn( $extra_meta, $post_being_indexed ) {

	// Bail fast if it's not the post type we're interested in.
	if ( 'product' !== get_post_type( $post_being_indexed ) ) {
		return $extra_meta;
	}

	// Retrieve product variations
	$product_variations_args = array(
		'post_type'       => 'product_variation',
		'posts_per_page'  => 100, // If you have more than 100 variations for each product, change this.
		'fields'          => 'ids',
		'post_parent'     => $post_being_indexed->ID,
		'suppress_filters' => false,
	);
	$product_variations      = get_posts( $product_variations_args );

	if ( ! empty( $product_variations ) ) {

		// Store all codes as a Custom Field.
		$extra_meta['m_product_isbns'] = array();

		// Loop through all product variations, grab and store the ISBN.
		foreach ( $product_variations as $product_variation ) {
			$extra_meta['m_product_isbns'][] = get_post_meta( absint( $product_variation ), '_isbn_code', true ); // Use the relevant meta key.
		}

	}

	// Make sure to return the newly found data.
	return $extra_meta;
}

add_filter( 'searchwp_extra_metadata', 'm_index_woocommerce_variation_isbn', 10, 2 );