如何獲取文檔 ID Firestore angular？

Question

我想更新firestore中文檔中的一個字段。 但是，文檔 ID 是必需的，並且我的所有文檔 ID 都是自動生成的。 我有一個名為 certName 的字段，它是唯一的，如何通過匹配 certName 字段來獲取文檔 ID？ 我已經嘗試了下面的代碼，但它顯示以下錯誤。

  updateDb(FILE_NAME,UPDATE_FIELD){
      const ref = this.afs.collection('history', ref => 
      ref.where('certName', '==', FILE_NAME));
      ref.snapshotChanges().pipe(
       map(actions => actions.map(a => {                                                      
         const data = a.payload.doc.data();
         const id = a.payload.doc.id;
         return { id };
  }))).subscribe((_doc: any) => {
   let id = _doc[0].payload.doc.id; //first result of query [0]
   this.afs.doc(`options/${id}`).update({transactionId: UPDATE_FIELD});
  })

這是錯誤

ERROR TypeError: Cannot read property 'payload' of undefined
at SafeSubscriber._next (db-service.service.ts:58)
at SafeSubscriber.__tryOrUnsub (Subscriber.js:183)
at SafeSubscriber.next (Subscriber.js:122)
at Subscriber._next (Subscriber.js:72)
at Subscriber.next (Subscriber.js:49)
at MapSubscriber._next (map.js:35)
at MapSubscriber.next (Subscriber.js:49)
at Notification.observe (Notification.js:20)
at AsyncAction.dispatch (observeOn.js:25)
at angular-fire.js:54

Answer 1

 updateDb(FILE_NAME,UPDATE_FIELD) {
     let docID;
     this.afs.collection('history', ref => ref.where('certName', '==', FILE_NAME)).valueChanges().subscribe(res => docID = res.id); 
     // (res.id) - id is the document id, the document id should be stored in the collection as a seperate field
     this.afs.collection('options').doc(docID).update({transactionId: UPDATE_FIELD});
  }