mutation.rb

class Mutations::SigninUser < GraphQL::Function
  SigninPayload = GraphQL::ObjectType.define do
    name 'SigninPayload'

    field :token, types.String, description: 'A JWT representing the authenticated user'
    field :user, Types::User, description: 'The authenticated user
  end
  
  type SigninPayload

  def call(_, args, ctx)
    ...
    # This would be okay
    SigninPayload.struct(token: '1234', user: user)
    # This would fail, because :expiration is not a valid field of SigninPayload
    SigninPayload.struct(token: '1234', expiration: '20171012')
  end
end