public function save() {
		global $wpdb;
		$table_name = $wpdb->prefix . $this->_table_name;
		if ( $this->id ) {
			if ( ! $wpdb->update( $table_name, $this->serialized_values(), array( $this->_pk => $this->{$this->_pk} ) ) ) {
				$wpdb->insert( $table_name, $this->serialized_values() );
				$this->id = $wpdb->insert_id;
			}
		} else {
			$wpdb->insert( $table_name, $this->serialized_values() );
			$this->id = $wpdb->insert_id;
		}
	}