series.php

				$fields['seriesPart'] = [
                        'type' => \WPGraphQL\Types::int(),
                        'description' => __( 'The number of the post within its series.' ),
						'resolve' => function( \WP_Post $post ) {

                            $series_number = get_post_meta( $post->ID, '_series_part', true );
                            
                            return ! empty ( $series_number ) ? $series_mumber : null;
						},
                ];