Union types

Union types are similar to interfaces however, while interfaces dictate fields that must be common to all implementations, unions do not. Unions only represent a selection of allowed types and make no requirements on those types. Here’s a union, expressed in GraphQL Schema Definition Language (SDL):

union MediaItem = Audio | Video | Image

Whenever we return a MediaItem in our schema, we might get an Audio, a Video or an Image. Note that members of a union type need to be concrete object types; you cannot create a union type out of interfaces, other unions or scalars.

A good use case for unions would be on a search field. For example:

searchMedia(term: "strawberry") {
  ... on Audio {
    duration
  }
  ... on Video {
    thumbnailUrl
  }
  ... on Image {
    src
  }
}

Here, the searchMedia field returns [MediaItem!]!, a list where each member is part of the MediaItem union. So, for each member, we want to select different fields depending on which kind of object that member is. We can do that by using inline fragments.

Defining unions

In Strawberry there are two ways to define a union:

You can use the use the Union type from the typing module which will autogenerate the type name from the names of the union members:

from typing import Union
import strawberry

@strawberry.type
class Audio:
    duration: int

@strawberry.type
class Video:
    thumbnail_url: str

@strawberry.type
class Image:
    src: str

@strawberry.type
class Query:
    latest_media: Union[Audio, Video, Image]

union AudioVideoImage = Audio | Video | Image

type Query {
  latestMedia: AudioVideoImage!
}

type Audio {
  duration: Int!
}

type Video {
  thumbnailUrl: String!
}

type Image {
  src: String!
}

Or if you need to specify a name or a description for a union you can use the strawberry.union function:

import strawberry

@strawberry.type
class Query:
    latest_media: strawberry.union("MediaItem", types=(Audio, Video, Image))

union MediaItem = Audio | Video | Image

type Query {
  latest_media: AudioVideoImage!
}

type Audio {
  duration: Int!
}

type Video {
  thumbnailUrl: String!
}

type Image {
  src: String!
}

Resolving a union

When a field’s return type is a union, GraphQL needs to know what specific object type to use for the return value. In the example above, each MediaItem must be categorized as an Audio, Image or Video type. To do this you need to always return an instance of an object type from your resolver:

from typing import Union
import strawberry

@strawberry.type
class Query:
    @strawberry.field
    def latest_media(self) -> Union[Audio, Video, Image]:
        return Video(
            thumbnail_url="https://i.ytimg.com/vi/dQw4w9WgXcQ/hq720.jpg",
        )