@oneOf input objects (#1062, #1347)

[tyranron]

Resolves #1062
Part of #1347

Synopsis

GraphQL spec September 2025 supports @oneOf directive on input objects:

• RFC: OneOf Input Objects graphql/graphql-spec#825

OneOf Input Objects are a special variant of Input Objects where the type system asserts that exactly one of the fields must be set and non-null, all others being omitted. This is represented in introspection with the __Type.isOneOf: Boolean field, and in SDL via the @oneOf directive on the input object.

input UserBy @oneOf {
  id: ID
  email: String
  username: String
  registrationNumber: Int
}
type Query {
  user(by: UserBy!): User
}

Solution

• Add @oneOf directive to introspection schema
• Add __Type.isOneOf field to introspection schema
• Support placing @oneOf directive to the correct place
• Support enums in derive(GraphQLInputObject) macro

Checklist

• Docs are added
• Tests are added
• CHANGELOG entry is added