Interfaces & Unions

Interfaces and unions are GraphQL's abstract types. An interface defines a set of fields that a concrete Object type (or another interface) must include to implement it. A union has no fields of its own; it only lists which Object types a field may return.

Baeta supports both directly in your .gql files and infers __typename requirements automatically.

See the GraphQL spec for the underlying semantics.

Interfaces

Defining an interface

src/modules/comic/comic.gql

interface Readable {
  id: ID!
  pages: Int!
}

type Comic implements Readable {
  id: ID!
  title: String!
  year: Int!
  artist: String!
  pages: Int!
}

type Book implements Readable {
  id: ID!
  title: String!
  year: Int!
  author: String!
  pages: Int!
}

Unions

Defining a union

src/modules/media/media.gql

type Movie {
  id: ID!
  title: String!
  year: Int!
}

type TVShow {
  id: ID!
  title: String!
  year: Int!
  seasons: Int!
}

union Media = Movie | TVShow | Book

type Query {
  media: [Media!]!
}

Extending unions

Other modules can add new members to a union via extend:

extend union Media = Comic

Type resolution

Resolvers that return a union or interface must include __typename on every returned value. The generated types enforce this: omit __typename and the resolver fails to typecheck, so you never need a custom resolveType.

Resolvers

Each entry below carries its own __typename:

const mediaQuery = Query.media.resolve(() => {
  return [
    {
      __typename: "Book" as const,
      id: "1",
      title: "The Book",
      author: "Jon Doe",
      year: 2021,
      pages: 100,
    },
    {
      __typename: "Movie" as const,
      id: "2",
      title: "The Movie",
      year: 2022,
    },
    {
      __typename: "TVShow" as const,
      id: "3",
      title: "The TV Show",
      year: 2023,
      seasons: 1,
    },
    {
      __typename: "Movie" as const,
      id: "4",
      title: "Another Movie",
      year: 2023,
    },
  ];
});