<?php

/**
 * Prevent taxonomy-based attributes from being overwritten when a Product Set
 * updates a product already in your store.
 *
 * @see wc_get_product_terms()
 * @link http://stackoverflow.com/a/13454788/2489248
 *
 * @param array|string $value The current value of the $attribute for this $post.
 * @param string $attribute The slug of the attribute. Examples: pa_color or pa_shoe-size
 * @param array $post An array of post data including ID, post_title, post_status, etc...
 * @param array $product An array of product data returned from the Datafeedr API.
 * @param array $set A post array for this Product Set with an array key of postmeta containing all post meta data.
 * @param string $action The action the Product Set is performing. Value are either "insert" or "update".
 *
 * @return array|string The updated attribute's value.
 */
add_filter( 'dfrpswc_filter_attribute_value', 'mycode_prevent_attribute_override_on_update', 100, 6 );
function mycode_prevent_attribute_override_on_update( $value, $attribute, $post, $product, $set, $action ) {

	/**
	 * Add slug value for each attribute that should not be modified during a Product Set update.
	 * Slugs can be found here: WordPress Admin Area > Products > Attributes
	 */
	$attrs = array(
		'color',
		'size',
	);

	if ( 'insert' == $action ) {
		return $value;
	}

	foreach ( $attrs as $attr ) {

		$name = ( substr( $attr, 0, 3 ) === 'pa_' ) ? $attr : 'pa_' . $attr;

		if ( $name == $attribute ) {
			$value = wc_get_product_terms( $post['ID'], $name, array( 'fields' => 'names' ) );
			$value = implode( WC_DELIMITER, $value );
		}
	}

	return $value;
}