wp-graphql-food-pizza-cake-scenario-1.php

add_action( 'graphql_register_types', function() {

	register_graphql_field( 'Food', 'price', [
		'type' => 'String',
		'description' => __( 'The cost of the food item', 'your-textdomain' ),
		'resolve' => function( $food ) {
			return get_post_meta( $food->databaseId, 'price', true );
		}
	]);

	register_graphql_object_type( 'Pizza', [
		'description' => __( 'A tasty food best prepared in an air-fryer', 'your-textdomain' ),
		'interfaces' => [ 'Food' ],
		'model' => WPGraphQL\Model\Post::class,
		'eagerlyLoadType' => true,
		'fields' => [
			'toppings' => [
				'type' => [ 'list_of' => 'String' ],
				'resolve' => function( $pizza ) {
					$toppings = get_post_meta( $pizza->databaseId, 'topping', false );

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

	register_graphql_object_type( 'Cake', [
		'description' => __( 'A tasty dessert, most likely also good if heated in an air-fryer', 'your-textdomain' ),
		'interfaces' => [ 'Food' ],
		'eagerlyLoadType' => true,
		'model' => WPGraphQL\Model\Post::class,
		'fields' => [
			'frostingColor' => [
				'type' => 'String',
				'resolve' => function( $cake ) {
					return get_post_meta( $cake->databaseId, 'frosting_color', true );
				}
			],
		]
	]);

});

add_action( 'init', function() {

	register_post_type( 'food', [
		'public' => true,
		'label' => 'Food',
		'show_in_graphql' => true,
		'supports' => [ 'title', 'editor', 'custom-fields' ],
		'graphql_single_name' => 'Food',
		'graphql_plural_name' => 'Food',
		'graphql_kind' => 'interface',
		'graphql_resolve_type' => function( $food ) {
			$food_type = get_post_meta( $food->databaseId, 'food_type', true );

			if ( 'pizza' === $food_type ) {
				$return_type = 'Pizza';
			} else {
				$return_type = 'Cake';
			}

			return $return_type;
		},
	] );

});