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

解决办法是:

在firebase文档中编写一个计数器，每次创建新条目时都在事务中增加计数器

您将计数存储在新条目的字段中(即:position: 4)。

然后在该字段上创建一个索引(position DESC)。

您可以对查询执行跳过+限制操作。Where("position"， "<" x).OrderBy("position"， DESC)

希望这能有所帮助!

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

FireStore现在支持该功能，尽管是Beta版。 下面是Firebase的官方文档

var variable=0
variable=variable+querySnapshot.count

那么如果你要在String变量上使用它，那么

let stringVariable= String(variable)

Firestore引入了一个新的query .count()，它获取查询的计数而不获取文档。

这将允许简单地查询所有集合项并获得该查询的计数。

Ref:

Firebase 10 iOS SDK [JS SDK PR] (https://github.com/firebase/firebase-js-sdk/pull/6608)