# Unions

GraphQL unions can be defined by placing a `@gqlUnion` docblock directly before a:

-   Type alias of a union of object types
-   Type alias of reference to a single object type (for unions with one member)

```ts
/**
 * A description of my union.
 * @gqlUnion <optional name of the union, if different from type name>
 */
type MyUnion = User | Post;

/** @gqlType */
class User {
  __typename: "User";
  /** @gqlField */
  name: string;
}

/** @gqlType */
class Post {
  __typename: "Post";
  /** @gqlField */
  content: string;

  /** @gqlField */
  author: User;
}
```

> **NOTE:**
> All the types referenced in the TypeScript union but be explicitly annotated with `@gqlType`. Grats will remind you with a helpful error message if you forget.

> **NOTE:**
> Grats must be able to determine the typename of any type which members of a union. To achieve this Grats will validate that all member types either define a `__typename: "MyType" as const` property or are exported classes. Grats can use either to determin the typename at runtime.