WP_REST_Controller::add_additional_fields_to_object

	 *
	 * @param object          $object  Data model like WP_Term or WP_Post.
	 * @param WP_REST_Request $request Full details about the request.
	 * @return bool|WP_Error True on success, WP_Error object if a field cannot be updated.
	 */
	protected function update_additional_fields_for_object( $object, $request ) {
		$additional_fields = $this->get_additional_fields();

		foreach ( $additional_fields as $field_name => $field_options ) {
			if ( ! $field_options['update_callback'] ) {
				continue;
			}

			// Don't run the update callbacks if the data wasn't passed in the request.
			if ( ! isset( $request[ $field_name ] ) ) {
				continue;
			}

			$result = call_user_func( $field_options['update_callback'], $request[ $field_name ], $object, $field_name, $request, $this->get_object_type() );