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。

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

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

截至2022年10月，Firestore在客户端sdk上引入了count()方法。现在您可以在没有下载的情况下计算查询。

对于1000份文件，它将收取你阅读1份文件的费用。

网络(v9)

Firebase 9.11.0介绍:

const collectionRef = collection(db, "cities");
const snapshot = await getCountFromServer(collectionRef);
console.log('count: ', snapshot.data().count);

Web V8

不可用。

节点(管理)

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

Android (Kotlin)

在firestore v24.4.0 (BoM 31.0.0)引入:

val query = db.collection("cities")
val countQuery = query.count()
countQuery.get(AggregateSource.SERVER).addOnCompleteListener { task ->
    if (task.isSuccessful) {
        val snapshot = task.result
        Log.d(TAG, "Count: ${snapshot.count}")
    } else {
        Log.d(TAG, "Count failed: ", task.getException())
    }
}

苹果平台(Swift)

Firestore v10.0.0引入:

do {
  let query = db.collection("cities")
  let countQuery = query.countAggregateQuery
  let snapshot = try await countQuery.aggregation(source: AggregateSource.server)
  print(snapshot.count)
} catch {
  print(error)
}

不，目前还没有内置的聚合查询支持。然而，有几件事你可以做。

这里记录了第一个。您可以使用事务或云函数来维护聚合信息:

这个例子展示了如何使用一个函数来跟踪子集合中的评级数量，以及平均评级。

exports.aggregateRatings = firestore
  .document('restaurants/{restId}/ratings/{ratingId}')
  .onWrite(event => {
    // Get value of the newly added rating
    var ratingVal = event.data.get('rating');

    // Get a reference to the restaurant
    var restRef = db.collection('restaurants').document(event.params.restId);

    // Update aggregations in a transaction
    return db.transaction(transaction => {
      return transaction.get(restRef).then(restDoc => {
        // Compute new number of ratings
        var newNumRatings = restDoc.data('numRatings') + 1;

        // Compute new average rating
        var oldRatingTotal = restDoc.data('avgRating') * restDoc.data('numRatings');
        var newAvgRating = (oldRatingTotal + ratingVal) / newNumRatings;

        // Update restaurant info
        return transaction.update(restRef, {
          avgRating: newAvgRating,
          numRatings: newNumRatings
        });
      });
    });
});

jbb提到的解决方案在您只想不频繁地计数文档时也很有用。确保使用select()语句来避免下载所有文档(当您只需要一个计数时，这是很大的带宽)。select()目前仅在服务器sdk中可用，因此该解决方案不适用于移动应用程序。

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)