public function save_data( $old_session_key = 0 ) {
		// Dirty if something changed - prevents saving nothing new.
		if ( $this->_dirty && $this->has_session() ) {
			global $wpdb;

			$wpdb->query(
				$wpdb->prepare(
					"INSERT INTO {$wpdb->prefix}woocommerce_sessions (`session_key`, `session_value`, `session_expiry`) VALUES (%s, %s, %d)
 					ON DUPLICATE KEY UPDATE `session_value` = VALUES(`session_value`), `session_expiry` = VALUES(`session_expiry`)",
					$this->_customer_id,
					maybe_serialize( $this->_data ),
					$this->_session_expiration
				)
			);

			wp_cache_set( $this->get_cache_prefix() . $this->_customer_id, $this->_data, WC_SESSION_CACHE_GROUP, $this->_session_expiration - time() );
			$this->_dirty = false;
			if ( get_current_user_id() != $old_session_key && ! is_object( get_user_by( 'id', $old_session_key ) ) ) {
				$this->delete_session( $old_session_key );
			}
		}
	}