如何使用事务监视整个子集合？ [英] How to monitor entire subcollection using transaction?

问题描述

按照此 answer 的操作，我尝试使用事务来实现，以监视整个Firestore子集合中是否添加了新文档。基本上，如果只有一个文档，我只想将新文档写入子集合。我需要使用事务来避免争用情况导致子集合中的文档> 2。最大值应为2。

Follow this answer I am try to implement using transaction to monitor entire Firestore subcollection for new document added. Basically I only want write new document to subcollection if there is only one document. I need use transaction to avoid race condition resulting in >2 document in subcollection. Max should be 2.

如何使用事务监视添加到子集合中的文档？

How to use transaction to monitor document added to subcollection?

我很努力是时候去做，但是无法解决。

I am try long time to do but cannot solve.

我正在实验通过子集合对文档进行迭代，但是不知道如何通过事务来完成。

I am experiment use iterate through subcollection for document but not know how to do this through transaction.

到目前为止我的代码（可能是错误的方法）：

My code so far (maybe wrong method):

                Firestore.instance.runTransaction((transaction) async {


                  final CollectionReference collectionRef = ref
                      .document(‘document’).collection(‘subCollection’);



                 List<DocumentSnapshot> subCollectionDocsSnapshot = [];
                  await collectionRef.getDocuments().then((querySnapshot) =>
                      querySnapshot.documents.forEach((document) {
                        subCollectionDocsSnapshot.add(document);
                      }
                      ));


                  final DocumentReference newDocRef = collectionRef.document(docName);

                  await transaction.set(
                      newDocRef,
                      {‘docName’: docName,
                      }
                  );

                });

如何解决？

谢谢！

更新：

我也尝试添加 transaction.get（）遍历子集合文档，但对竞争条件没有影响：

I have try add also transaction.get() to iterate through subcollection docs but it have no effect on race condition:

                  subCollectionDocsSnapshot.forEach((document) {
                    transaction.get(document.reference);
                  });

推荐答案

Firestore交易不支持此功能。在事务中，您只能按其ID查找文档。您无法处理集合的全部内容，并且如果在交易过程中添加了新文档，则必须重试该交易。您也不能根据查询的结果进行交易。

This isn't supported by Firestore transactions. Within a transaction, you can only find a document by its ID. You can't transact on the entire contents of a collection, and have that transaction retry if an new document is added while in the middle of the transaction. You also can't transact on the results of a query.

相反，请考虑在另一个集合中计算一个集合中的文档数来使用另一个文档，然后使用在您的交易中。或者至少是一个记录布尔值的文档，该布尔值指示集合中是否包含> 2个文档。

Instead, consider having a different document in another collection that counts the number of documents in a collection, and use that in your transaction. Or, at the very least, a document that records a boolean indicating whether or not the collection has >2 documents.

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