Adding "Sale" product category to products that are on sale in Woocommerce

Updated 2 (October 2018)

save_post is a WordPress hook that works with $post_id argument and target all kind of posts. You need to target product custom WooCommerce post_type first in a condition (and publish post_status).

Also as it's not a post object you can't use is_on_sale() method with it. But you can use get_post_meta() function to check if the sale price is set in the product.

Here is the fully functional and tested code (for simple products only):

add_action( 'save_post_product', 'update_product_set_sale_cat' );function update_product_set_sale_cat( $post_id ) {    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {        return $post_id;    }    if ( ! current_user_can( 'edit_product', $post_id ) ) {        return $post_id;    }    if( get_post_status( $post_id ) == 'publish' && isset($_POST['_sale_price']) ) {        $sale_price = $_POST['_sale_price'];        if( $sale_price >= 0 && ! has_term( 'Sale', 'product_cat', $post_id ) ){            wp_set_object_terms($post_id, 'sale', 'product_cat', true );        }    }}

Code goes in function.php file of your active child theme (or active theme). Tested and works.

Related: Auto remove Sale product category from not on sale products in Woocommerce