Multiple return types for a field

I’m submitting a…

[ ] Regression 
[ ] Bug report
[ ] Feature request
[x] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead post your question on Stack Overflow.

Current behavior

I’ve got a class with multiple fields, and the one which returns object of union type results in an error: “Error: Cannot determine a GraphQL input type for the “users”. Make sure your class is decorated with an appropriate decorator.”

The issue is not about decorators from what I’ve researched, it is about the type I specify as a return type in @Field decorator.

Expected behavior

I need for the “access” field to be able to return an object of one out of multiple types.

Minimal reproduction of the problem with instructions

Field “users” in WidgetAccess class is the questionable part

BaseWidgetType - top level class from which the issue needs to start be reviewed from, "access" field is 
the one with problems inside it:

baseWidgetType.ts:
@ObjectType('BaseWidget')
export class BaseWidgetType extends BaseType {
  @Field((type) => BaseWidgetConfig, { nullable: true })
  widgetConfig: BaseWidgetConfig;

  @Field((type) => WidgetAccess, { nullable: true })
  access: WidgetAccess;
}


widgetAccess.ts:
export const WidgetUserUnion = createUnionType({
  name: 'WidgetUserUnion',
  types: () => [WidgetSingleUser, WidgetGroup],
});


@ObjectType('WidgetAccess')
@InputType('WidgetAccessInput')
export class WidgetAccess {
  @Field({ nullable: false })
  public: boolean;

  @Field((type) => [WidgetUserUnion], { nullable: true })
  users: typeof WidgetUserUnion[];
}


widgetUser.ts:
@ObjectType('WidgetUser')
@InputType('WidgetUserInput')
export class WidgetUser {
  @Field({ nullable: false })
  userId: string;
}


widgetGroup.ts:
@ObjectType('WidgetGroup')
@InputType('WidgetGroupInput')
export class WidgetGroup {
  @Field({ nullable: false })
  groupId: string;
}

What is the motivation / use case for changing the behavior?

It would be nice to be able to split differently structured units by different types, to restrict client to enter logically disjoint sets of data.

Environment

Nest version: 7.6.15

 
For Tooling issues:
- Node version: 14.17.0  
- Platform:  Windows 

[kamilmysliwiec]

Please provide a minimum reproduction repository.

[kamilmysliwiec]

https://github.com/graphql/graphql-spec/issues/488