Query.select() on relationships

"questions": {
      "total": 2,
      "documents": [
        {
          "order": 1,
          "$id": "quiz_question_1",
          "questionId": {
            "$id": "question_1",
            "$databaseId": "...",
            "$collectionId": "..."
          }
        },
        {
          "order": 2,
          "$id": "quiz_question_2",
          "questionId": {
            "$id": "question_2",
            "$databaseId": "...",
            "$collectionId": "..."
          }
        }
      ]
    }

Query.select(["question", "order"]);

AppwriteException: Cannot select attributes: questionId

code: 400,
type: 'general_query_invalid',
response: {
  message: 'Cannot select attributes: questionId',
  code: 400,
  type: 'general_query_invalid',
  version: '1.6.0'
}

Query.select(["questionId", "order"]);

const response = await database.listDocuments("<DATABASE_ID>", "<COLLECTION_ID>");
const documents = response.documents.map(doc => ({
  order: doc.order,
  questionId: doc.questionId
}));

console.log(documents);