Edit

Unions

Sometimes our API has to be flexible and return not a specific type but the one from the range of possible types. The example might be a movie site's search functionality: using the provided phrase we search in database not only for movies but also for actors, so the query has to return the list Movie or Actor types.

You can read more about GraphQL union type in official docs.

Usage

Let's start by creating the object types from example above:

@ObjectType()
class Movie {
  @Field()
  name: string;

  @Field()
  rating: number;
}

@ObjectType()
class Actor {
  @Field()
  name: string;

  @Field(type => Int)
  age: number;
}

Then we have to create the union type from the object types above:

import { createUnionType } from "type-graphql";

const SearchResultUnion = createUnionType({
  name: "SearchResult", // the name of the GraphQL union
  types: [Movie, Actor], // array of object types classes
});

All that's left to do is to use the union type in the query. Notice, that due to TypeScript's reflection limitation, you have to explicitly use SearchResultUnion value in @Query decorator return type annotation. For TS compile-time type safety you can also use typeof SearchResultUnion which is equal to type Movie | Actor.

@Resolver()
class SearchResolver {
  @Query(returns => [SearchResultUnion])
  async search(@Arg("phrase") phrase: string): Promise<Array<typeof SearchResultUnion>> {
    const movies = await Movies.findAll(phrase);
    const actors = await Actors.findAll(phrase);

    return [...movies, ...actors];
  }
}

Be aware that when your query/mutation return type (or field type) is an union, you have to return an specific instance of the object type class. Otherwise, graphql-js will not be able to detect the underlying GraphQL type correctly when you use plain JS objects.

Et Voilà! You can now build the schema and make the example query 😉

query {
  search(phrase: "Holmes") {
    ... on Actor {
      # maybe Katie Holmes?
      name
      age
    }
    ... on Movie {
      # for sure Sherlock Holmes!
      name
      rating
    }
  }
}