Filters the default WooCommerce product category shortcode to only include items on sale

gistfile1.php

add_filter( 'woocommerce_shortcode_products_query', 'wooninja_only_sale_items', 10, 2 );

function wooninja_only_sale_items( $args, $atts ) {
	if ( $atts['category'] ) {
		$product_ids_on_sale = wc_get_product_ids_on_sale();
		$args['post__in']	= array_merge( array( 0 ), $product_ids_on_sale );
	}
	return $args;
}