Example showing a connection from one Post Type to another, resolving with related IDs stored in Post Meta

posts-to-posts-connections-wp-graphql.php

add_action( 'init', function() {
	register_post_type( 'physician', [
		'hierarchical'        => false,
		'label'               => __( 'Physicians', 'wp-graphql' ),
		'public'              => true,
		'show_in_graphql'     => true,
		'graphql_single_name' => 'Physician',
		'graphql_plural_name' => 'Physicians',
	] );
	register_post_type( 'disease', [
		'hierarchical'        => false,
		'public'              => true,
		'label'               => __( 'Diseases', 'wp-graphql' ),
		'show_in_graphql'     => true,
		'graphql_single_name' => 'Disease',
		'graphql_plural_name' => 'Diseases',
	] );

} );

add_action( 'graphql_register_types', function() {

	register_graphql_connection( [
		'fromType'      => 'Physician',
		'toType'        => 'Disease',
		'fromFieldName' => 'diseases',
		'resolveNode'   => function( $id, $args, $context, $info ) {
			return \WPGraphQL\Data\DataSource::resolve_post_object( $id, $context );
		},
		'resolve'       => function( \WPGraphQL\Model\Post $source, $args, $context, $info ) {

			$diseases = get_post_meta( $source->ID, '_pods_disease', true );

			if ( empty( $diseases ) || ! is_array( $diseases ) ) {
				return null;
			}

			$resolver = new \WPGraphQL\Data\Connection\PostObjectConnectionResolver( $source, $args, $context, $info, 'disease' );
			$resolver->setQueryArg( 'post__in', array_values( $diseases ) );
			$resolver->setQueryArg( 'post_parent', null );
			$connection = $resolver->get_connection();

			return $connection;

		}
	] );

} );