firebase firestore cloud函数从不执行

我让FireStore可以在我有这样的数据库的地方运行

'posts/{postId}/comments/{commentId}'

以及在添加新注释时增加postid中字段的函数

函数已部署,但在添加新注释时从不执行

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

exports.comment= functions.firestore
    .document('posts/{postId}/comments/{commentId}')
    .onWrite((change, context) => {

   const commentId = event.params.commentId; 
    const postId = event.params.postId;
 const docRef = admin.firestore().collection('posts').doc(postId);
 return docRef.update({numberOfComments:200});

    });

我添加了很多评论,即使执行是0