，遍历整个集合太繁琐了.相反，您可以使用 query.stream().

as @Doug mentioned, iterating over the entire collection is too heavy. instead you can stream the query results and iterate over keys, using query.stream().

要访问和更新文档中的单个字段，请首先使用更新()，同时指定该字段.

to access and update a single field in a document, first retrieve the document by its ID with doc(), then use update() while specifying the field.

这是根据您的情况实现的示例.

here's an example of implementation based on your scenario.

package.json

{
  "dependencies": {
    "firebase-admin": "^6.5.1",
    "firebase-functions": "^2.1.0"
  }
}

index.js

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

admin.initializeApp(functions.config().firebase);

const studentsRef = admin.firestore().collection('students');
const dashboardRef = admin.firestore().collection('dashboard');

exports.addStudent = functions.firestore
  .document('students/{studentId}')
  .onCreate((snap, context) => {
    var newStudent = snap.data();
    console.log('New student in collection: ', newStudent);

    var activeCount = 0;
    studentsRef.where('status', '==', true).select().stream()
      .on('data', () => {
        ++activeCount;     
      }).on('end', () => { 
        dashboardRef.where('type', '==', 'students').get()
          .then(querySnap => {
            if (querySnap.docs[0].data().count == activeCount){
              console.log('No new active student: ', querySnap.docs[0].data());
            } else {
                console.log('New active count: ', activeCount);
                console.log('Student Dashboard before update: ', querySnap.docs[0].id, '=>', querySnap.docs[0].data());
                dashboardRef.doc(querySnap.docs[0].id).update({
                  count: activeCount
                });
                console.log('Active student count updated: ', querySnap.docs[0].data().count, '=>', activeCount);
              };
            });
        });
  return null
  });

gcloud

gcloud functions deploy addStudent \
  --runtime nodejs8 \
  --trigger-event providers/cloud.firestore/eventTypes/document.create \
  --trigger-resource "projects/[PROJECT_ID]/databases/(default)/documents/students/{studentId}"