Gamote/nestjs-graphql-union-mrr

Nest.js GraphQL Union Issue Minimum Reproduction Repo

Tech used

• Nest.js v8.0.0
• Nest.js GraphQL v10.0.10 (Gateway + Federation)
• Mercurius v9.4.0 (with Fastify)

Issue description

TL;DR: References part of a union type can't be resolved by the GraphQL Gateway (federation) when is used as a type for a field. If the union type is used as return type for a Query or Mutation all it works as expected.

As you know, when working with GraphQL Federation one service can return a type reference (e.g. { __typename: 'Song', id: '1' }) that can later be resolved by another service. In this service we have created a union type called FavoriteItemUnion that contains the Song model (extension of the Song type defined in the songs service) and the Book model (extension of the Book type defined in the books service).

Working example

When FavoriteItemUnion is used as return type for a Query, GraphQL is able to retrieve the data from the appropriate service.

// Query resolver defined in the `users` service
@Query(() => FavoriteItemUnion)
async topItem() {
  return { __typename: 'Song', id: 3 };
}

# Query to retrieve the top item
query TopItemQuery {
  topItem {
    ... on Song {
      id
      title
    }
  }
}

// Query result
{
  "data": {
    "topItem": {
      "id": 3,
      "title": "Song 3"
    }
  }
}

Issue example

When FavoriteItemUnion is used as return type for a field in an Object type.

// Field definition in the `users` service
@ObjectType()
@Directive('@key(fields: "id")')
export class User {
  @Field(() => Int)
  id: number;

  @Field()
  firstName: string;

  @Field()
  lastName: string;
  
  // This field returns a union type
  @Field(() => FavoriteItemUnion)
  favoriteItem: typeof FavoriteItemUnion;
}

// Query resolver defined in the `users` service
@Query(() => User)
async getUserById(@Args('id') id: number) {
  return this.usersService.getById(id);
  /*
    ^ For `userId=1` will return the following:
    {
      "id": 1,
      "firstName": "John",
      "lastName": "Doe",
      "favoriteItem": {
        "__typename": "Song",
        "id": 1
      }
    }
  */
}

# Query to retrieve the user + his favorite item
query GetUserById {
  getUserById(id: 1) {
    id
    firstName
    lastName
    favoriteItem {
      ... on Song {
        id
        title
      }
    }
  }
}

{
  "errors": [
    {
      "message": "Cannot return null for non-nullable field Song.title.",
      "locations": [
        {
          "line": 9,
          "column": 9
        }
      ],
      "path": [
        "getUserById",
        "favoriteItem",
        "title"
      ]
    }
  ],
  "data": null
}

In this case, if we remove the title from the query, we will get a response as GraphQL doesn't need to resolve any more data.

How to run

1. Install dependencies

   $ yarn

2. Start the federated services

   # Users service
   $ yarn start users
   
   # Songs service
   $ yarn start songs
   
   # Books service
   $ yarn start books

3. Start the gateway

   $ yarn start

Now you can visit the gateway at http://localhost:3010/graphql and use the queries described above. ``