有了新版本的Firebase，您现在可以运行聚合查询了! 简单的写

.count().get(); 

在您的询问之后。

目前，firebase只允许服务器端计数，如下所示

const collectionRef = db.collection('cities');
const snapshot = await collectionRef.count().get();
console.log(snapshot.data().count);

请不要，这是为nodeJS

除了上面我的npm包adv-firestore-functions，你还可以使用firestore规则来强制一个好的计数器:

Firestore规则

function counter() {
  let docPath = /databases/$(database)/documents/_counters/$(request.path[3]);
  let afterCount = getAfter(docPath).data.count;
  let beforeCount = get(docPath).data.count;
  let addCount = afterCount == beforeCount + 1;
  let subCount = afterCount == beforeCount - 1;
  let newId = getAfter(docPath).data.docId == request.path[4];
  let deleteDoc = request.method == 'delete';
  let createDoc = request.method == 'create';
  return (newId && subCount && deleteDoc) || (newId && addCount && createDoc);
}

function counterDoc() {
  let doc = request.path[4];
  let docId = request.resource.data.docId;
  let afterCount = request.resource.data.count;
  let beforeCount = resource.data.count;
  let docPath = /databases/$(database)/documents/$(doc)/$(docId);
  let createIdDoc = existsAfter(docPath) && !exists(docPath);
  let deleteIdDoc = !existsAfter(docPath) && exists(docPath);
  let addCount = afterCount == beforeCount + 1;
  let subCount = afterCount == beforeCount - 1;
  return (createIdDoc && addCount) || (deleteIdDoc && subCount);
}

像这样使用它们:

match /posts/{document} {
  allow read;
  allow update;
  allow create: if counter();
  allow delete: if counter();
}
match /_counters/{document} {
  allow read;
  allow write: if counterDoc();
}

前端

用以下函数替换你的set和delete函数:

set

async setDocWithCounter(
  ref: DocumentReference<DocumentData>,
  data: {
    [x: string]: any;
  },
  options: SetOptions): Promise<void> {

  // counter collection
  const counterCol = '_counters';

  const col = ref.path.split('/').slice(0, -1).join('/');
  const countRef = doc(this.afs, counterCol, col);
  const countSnap = await getDoc(countRef);
  const refSnap = await getDoc(ref);

  // don't increase count if edit
  if (refSnap.exists()) {
    await setDoc(ref, data, options);

    // increase count
  } else {
    const batch = writeBatch(this.afs);
    batch.set(ref, data, options);

    // if count exists
    if (countSnap.exists()) {
      batch.update(countRef, {
        count: increment(1),
        docId: ref.id
      });
      // create count
    } else {
      // will only run once, should not use
      // for mature apps
      const colRef = collection(this.afs, col);
      const colSnap = await getDocs(colRef);
      batch.set(countRef, {
        count: colSnap.size + 1,
        docId: ref.id
      });
    }
    batch.commit();
  }
}

删除

async delWithCounter(
  ref: DocumentReference<DocumentData>
): Promise<void> {

  // counter collection
  const counterCol = '_counters';

  const col = ref.path.split('/').slice(0, -1).join('/');
  const countRef = doc(this.afs, counterCol, col);
  const countSnap = await getDoc(countRef);
  const batch = writeBatch(this.afs);

  // if count exists
  batch.delete(ref);
  if (countSnap.exists()) {
    batch.update(countRef, {
      count: increment(-1),
      docId: ref.id
    });
  }
  /*
  if ((countSnap.data() as any).count == 1) {
    batch.delete(countRef);
  }*/
  batch.commit();
}

更多信息请看这里…

J

我同意@Matthew的观点，如果你执行这样的查询，成本会很高。

[开发者开始项目前的建议]

由于我们在一开始就预见到了这种情况，因此实际上可以用一个文档创建一个集合，即计数器，将所有计数器存储在一个类型为number的字段中。

例如:

对于集合上的每个CRUD操作，更新计数器文档:

当你创建一个新的集合/子集合时:(在计数器中+1)[1写操作] 当你删除一个集合/子集合时:(-1在计数器中)[1写操作] 当你更新一个现有的集合/子集合时，在counter文档上什么都不做:(0) 当你读取一个现有的集合/子集合时，在计数器文档上什么都不做:(0)

下一次，当您想要获得集合的数量时，您只需要查询/指向文档字段。[1读操作]

此外，你可以将集合的名称存储在一个数组中，但这将是棘手的，数组在firebase中的条件如下所示:

// we send this
['a', 'b', 'c', 'd', 'e']
// Firebase stores this
{0: 'a', 1: 'b', 2: 'c', 3: 'd', 4: 'e'}

// since the keys are numeric and sequential,
// if we query the data, we get this
['a', 'b', 'c', 'd', 'e']

// however, if we then delete a, b, and d,
// they are no longer mostly sequential, so
// we do not get back an array
{2: 'c', 4: 'e'}

所以，如果你不打算删除集合，你实际上可以使用数组来存储集合名称的列表而不是每次都查询所有的集合。

希望能有所帮助!

自9.11.0版本以来，有一个新的内置函数getCountFromServer()，它在不实际下载文档的情况下获取结果集中的文档数量。

https://firebase.google.com/docs/reference/js/firestore_#getcountfromserver

firebaseFirestore.collection("...").addSnapshotListener(new EventListener<QuerySnapshot>() {
        @Override
        public void onEvent(QuerySnapshot documentSnapshots, FirebaseFirestoreException e) {

            int Counter = documentSnapshots.size();

        }
    });