public function __isset( $key ) {
		$valid = array(
			'id',
			'product_attributes',
			'visibility',
			'sale_price_dates_from',
			'sale_price_dates_to',
			'post',
			'download_type',
			'product_image_gallery',
			'variation_shipping_class',
			'shipping_class',
			'total_stock',
			'crosssell_ids',
			'parent',
		);
		if ( $this->is_type( 'variation' ) ) {
			$valid = array_merge( $valid, array(
				'variation_id',
				'variation_data',
				'variation_has_stock',
				'variation_shipping_class_id',
				'variation_has_sku',
				'variation_has_length',
				'variation_has_width',
				'variation_has_height',
				'variation_has_weight',
				'variation_has_tax_class',
				'variation_has_downloadable_files',
			) );
		}
		return in_array( $key, array_merge( $valid, array_keys( $this->data ) ) ) || metadata_exists( 'post', $this->get_id(), '_' . $key ) || metadata_exists( 'post', $this->get_parent_id(), '_' . $key );
	}