Firebase云功能-快照.forEach(…)。则不是函数

我的数据库布局是:

@posts
   @postId_abc
     -total_score: ... // will update from cloud function
     -score_count: ... // will update from cloud function

@scores
   @postId_abc
      -uid_1: 10
      -uid_2: 20
      _uid_3: 50

cloud function 把所有的分数加起来 分数 参考并将其设置为 total_score 那个岗位上的财产。当我尝试下面的代码时,会出现错误:

FIREBASE WARNING: Exception was thrown by user callback. TypeError: snapshot.forEach(...).then is not a functio Exception from a finished function: TypeError: snapshot.forEach(...).then is not a function

看来我的 snapshot.forEach((child) => { ... }).then(() => { scoreCountProperty.set(...)

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();

exports.updateScore = functions.https.onCall((data, response) => {

    const postId = data.postId;

    const postsRef = admin.database().ref('/posts/' + postId);
    const scoreCountProperty = admin.database().ref('/posts/' + postId + '/' + 'score_count');

    var totalScore = 0.0;

    admin.database().ref('scores').child(postId).once('value', snapshot => {

    if (snapshot.exists()) {

        snapshot.forEach((child) => {

            totalScore += child.val()

        })
        .then(() => { 
                
            console.log('totalScore: ', totalScore);

            return postsRef.set({ "total_score": totalScore });       
        })
        .then(() => { 
                
            return scoreCountProperty.set(admin.database.ServerValue.increment(1));                
        })
        .catch((error) => {
            console.log('ERROR - updateScore Failed: ', error);
        });
    });
});