firebase - Firestore 事务失败，出现 "Transaction failed all retries"

Question

我正在运行一个非常简单的 Firestore 事务，该事务会检查文档是否存在，如果不存在则写入它。

(用例正在注册用户名 - 如果尚未注册，则当前用户可以获取它)

以下是相关 Flutter 代码的片段:

DocumentReference usernameDocRef =
    Firestore.instance.collection(_USERNAMES).document(username);

await Firestore.instance.runTransaction((transaction) async {
  var snapshot = await transaction.get(usernameDocRef);
  if (!snapshot.exists) {
    transaction.set(usernameDocRef, {
      _UsernamesKey.userid: _user.id,
    });
  }
});

这失败了，出现“事务所有重试失败”异常。

基于 Firestore documentation , 失败可能有两个原因:

• The transaction contains read operations after write operations. Read operations must always come before any write operations.
• The transaction read a document that was modified outside of the transaction. In this case, the transaction automatically runs again. The transaction is retried a finite number of times.

我认为我不会触发其中任何一个。有什么建议吗？

Answer 1

example transaction in the documentation在调用 update 时使用 await。也许你在调用 set 时需要同样的:

await Firestore.instance.runTransaction((transaction) async {
  var snapshot = await transaction.get(usernameDocRef);
  if (!snapshot.exists) {
    await transaction.set(usernameDocRef, {
      _UsernamesKey.userid: _user.id,
    });
  }
});