WooCommerce Related Products: What and How to Improve Them

functions.php

<?php
/**
 * @snippet       Get Related Products by Attributes
 * @author        Nicola Mustone
 * @author_url    https://nicolamustone.blog/2023/12/21/improve-woocommerce-related-products-recommendations/
 * @tested-up-to  WooCommerce 8.4.X
 */
add_filter( 'woocommerce_related_products', 'nm_realted_products_by_attributes', 10, 3 );
function nm_realted_products_by_attributes( $related_posts, $product_id, $args ) {
    $product = wc_get_product( $product_id );
	
	$attributes           = array( 'pa_attribute_1', 'pa_attribute_2', 'pa_attribute_3' );
	$new_related_products = array();
	
	foreach( $attributes as $attribute ) {
		$terms = $product->get_attribute( $attribute );
		
		if ( ! empty( $terms ) ) {
			$new_related_products = array_merge( $new_related_products, nm_get_related_products( $product_id, $attribute, $terms ) );
		}
	}
	
	if ( count( $new_related_products ) < 4 ) {
		$related_posts = array_slice( array_merge( $new_related_products, $related_posts ), 0, 4 );
	} else {
		$related_posts = array_slice( $new_related_products, 0, 4 );
	}

    return $related_posts;
}

function nm_get_related_products( $product_id, $attribute, $terms ) {
	$args = array(
		'post_type'      => 'product',
		'posts_per_page' => 4,
		'post__not_in'   => array( $product_id ),
		'orderby'        => 'rand',
		'tax_query'      => array(
			array(
				'taxonomy' => $attribute,
				'field'    => 'name',
				'terms'    => array_map( 'trim', explode( ',', $terms ) ),
			),
		),
	);

	$query            = new WP_Query( $args );
	$related_products = wp_list_pluck( $query->posts, 'ID' );

    return $related_products;
}