wp-graphql-food-pizza-cake-scenario-2.php

add_action( 'graphql_register_types', function() {

	register_graphql_interface_type( 'Food', [
		'description' => __( 'An item of food for sale', 'your-textdomain' ),
		'interfaces' => [ 'Node', 'NodeWithTitle' ],
		'fields' => [
			'price' => [
				'type' => 'String',
				'description' => __( 'The cost of the food item', 'your-textdomain' ),
				'resolve' => function( $food ) {
					return get_post_meta( $food->databaseId, 'price', true );
				},
			],
		],
		'resolveType' => function( $node ) {
			// use the post_type to determine what GraphQL Type should be returned. Default to Cake
			return get_post_type_object( $node->post_type )->graphql_single_name ?: 'Cake';
		}
	]);

	register_graphql_connection([
		'fromType' => 'RootQuery',
		'toType' => 'Food',
		'fromFieldName' => 'allFood',
		'resolve' => function( $root, $args, $context, $info ) {
			$resolver = new \WPGraphQL\Data\Connection\PostObjectConnectionResolver( $root, $args, $context, $info );
			$resolver->set_query_arg( 'post_type', [ 'pizza', 'cake' ] );
			return $resolver->get_connection();
		}
	]);

});

add_action( 'init', function() {

	register_post_type( 'pizza', [
		'public' => true,
		'label' => 'Pizza',
		'show_in_graphql' => true,
		'supports' => [ 'title', 'editor', 'custom-fields' ],
		'graphql_single_name' => 'Pizza',
		'graphql_plural_name' => 'Pizza',
		'graphql_interfaces' => [ 'Food' ],
		'graphql_fields' => [
			'toppings' => [
				'type' => [ 'list_of' => 'String' ],
				'resolve' => function( $pizza ) {
					$toppings = get_post_meta( $pizza->databaseId, 'topping', false );

					return is_array( $toppings ) ? $toppings : null;
				}
			],
		],
	] );

	register_post_type( 'cake', [
		'public' => true,
		'label' => 'Cake',
		'show_in_graphql' => true,
		'supports' => [ 'title', 'editor', 'custom-fields' ],
		'graphql_single_name' => 'Cake',
		'graphql_plural_name' => 'Cakes',
		'graphql_interfaces' => [ 'Food' ],
		'graphql_fields' => [
			'frostingColor' => [
				'type' => 'String',
				'resolve' => function( $cake ) {
					return get_post_meta( $cake->databaseId, 'frosting_color', true );
				}
			],
		]
	] );

});

This is snippet shows how to register 2 post types "Pizza" and "Cake" and have them "show_in_graphql". Then, register a connection to query allFood, allowing for a single query to return a list of Pizza and Cake together.

You can read more about this, at length on the WPGraphQL Blog.