Saves taxonomy terms to product meta in a comma separated list

TagToMeta.php

<?php

namespace UniversalYums\TagToMeta;

class TagToMeta {

	/**
	 * The single instance of the class.
	 */
	protected static $instance;

	/**
	 * Main TagToMeta Instance.
	 *
	 * Ensures only one instance of the TagToMeta is loaded or can be loaded.
	 *
	 * @return TagToMeta - Main instance.
	 */
	public static function instance() {
		if ( is_null( self::$instance ) ) {
			self::$instance = new self();
		}

		return self::$instance;
	}

	public function __construct() {
		add_action( 'woocommerce_update_product', array( $this, 'update_product' ) );
	}

	/**
	 * On product save, let's store taxonomy terms as post meta.
	 *
	 * @param $product_id
	 */
	public function update_product( $product_id ) {
		$taxonomies = array(
			'product_tag'     => 'tag',
			'product-country' => 'country',
		);

		foreach ( $taxonomies as $taxonomy => $meta_key ) {
			$terms = wp_get_post_terms( $product_id, $taxonomy );

			if ( ! $terms ) {
				delete_post_meta( $product_id, $meta_key );
			} else {
				$term_names = implode(
					',',
					array_map(
						static function ( $el ) {
							return $el->slug;
						},
						$terms
					)
				);

				update_post_meta( $product_id, $meta_key, $term_names );
			}
		}
	}
}