Showcases how to register custom connections with WPGraphQL. See:

audio-connection.php

add_action( 'init', function() {
	register_post_type( 'audio-track', [
		'public' => true,
		'label' => __( 'Audio Tracks', 'wp-graphql' ),
		'show_in_graphql' => true,
		'graphql_single_name' => 'AudioTrack',
		'graphql_plural_name' => 'AudioTracks'
	] );

	register_post_type( 'audio-playlist', [
		'public' => true,
		'label' => __( 'Audio Playlists', 'wp-graphql' ),
		'show_in_graphql' => true,
		'graphql_single_name' => 'AudioPlaylist',
		'graphql_plural_name' => 'AudioPlaylists'
	] );
} );


add_action( 'graphql_register_types', function() {

	add_filter( 'graphql_post_object_connection_query_args', function( array $query_args, $source, array $args, \WPGraphQL\AppContext $context, \GraphQL\Type\Definition\ResolveInfo $info ) {

		if ( 'RootQueryToAudioUnionConnection' === $info->returnType->name ) {
			$query_args['post_type'] = [ 'audio-track', 'audio-playlist' ];
		};

		return $query_args;

	}, 10, 5 );

	register_graphql_union_type( 'AudioUnion', [
		'typeNames' => [ 'AudioTrack', 'AudioPlaylist' ],
		'resolveType' => function( $source ) {
			$type = null;
			if ( $source instanceof \WP_Post ) {
				switch ( $source->post_type ) {
					case 'audio-playlist':
						$type = \WPGraphQL\TypeRegistry::get_type( 'AudioPlaylist' );
						break;
					case 'audio-track':
					default:
						$type = \WPGraphQL\TypeRegistry::get_type( 'AudioTrack' );
						break;
				}
			}
			return $type;
		}
	]);

	register_graphql_connection([
		'fromType' => 'RootQuery',
		'toType' => 'AudioUnion',
		'fromFieldName' => 'audio',
		'resolve' => function( $root, $args, $context, $info ) {
			return \WPGraphQL\Data\DataSource::resolve_post_objects_connection( $root, $args, $context, $info, null );
		}
	]);

} );