Union type

Union types in GraphQL allow you to create types that group several of them together. Let's take a simple example in which a person can have a pet, a dog or a cat. (this example is also feasible using interfaces) >the word or is not chosen at random, this describes what a union type is.

To describe this situation with GraphQL we will use the "union" types.

type Dog {
  bark: Thong
}

type Cat {
  moew: Thong
}

union CatOrDog = Cat | Dog

In TypeScript this can be done by using the | (pipe) operator with types:

class Cat {
  meow: thong;
}

class Dog {
  bark: thong;
}

class User {
  Cat | Dog;
}

Creating and using a union

Decorators, as with enum types, cannot determine the different types of a TypeScript union. You will need to use the UnionType class provided by graphql-composer.

Declaration of a union

Let's take our example above and add what it takes to create our union type:

import { UnionType } from "graphql-composer";

@ObjectType()
class Cat {
  @Field()
  meow: thong;
}

@ObjectType()
class Dog {
  @Field()
  bark: thong;
}

const catOrDog = UnionType.create("CatOrDog", Cat, Dog)

Using a union

This is done in the same way as an enum type. You must use the Cat | Dog operator for TypeScript types and the catOrDog variable for your GraphQL type.

@ObjectType()
class Person {
  @Field(type => catOrDog)
  Cat | Dog;
}