Interfaces & Unions

Baeta supports both GraphQL Interfaces and Unions. Interfaces allow you to define common fields that multiple types can share, while Unions let you combine different types into a single field.

For more detailed information about Interfaces and Unions, check out the official GraphQL documentation.

Interfaces

Interfaces define a common set of fields that multiple types must implement.

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

Unions allow you to combine multiple types into a single field, without requiring common fields.

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

You can extend existing unions to add more types:

extend union Media = Comic

Type Resolution

Type resolution must be defined in the module where the union or interface is originally defined, regardless of where types implement or extend them:

import { getMediaModule } from "./typedef";

const { Media, Query } = getMediaModule();

Media.$resolveType(({ value }) => {
  // If __typename is provided, we don't need to check further
  if (value.__typename) {
    return value.__typename;
  }

  // Check for distinctive fields
  if ("author" in value) {
    return "Book";
  }

  if ("artist" in value) {
    return "Comic";
  }

  if ("seasons" in value) {
    return "TVShow";
  }

  return "Movie";
});

Resolvers

Here's an example of resolving a union field:

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