Javascript 如何仅在将子项添加到数据库时触发云函数？

如何修改下面的代码，使函数仅在添加子项时触发？当前，每次指定引用发生更改时都会触发该函数

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

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

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



exports.myFunction = functions.database.ref('/messaging/{pushId}')

.onWrite(event => {

//child is added. Do something!

});

使用处理程序而不是

onWrite（）

exports.myFunction = functions.database.ref('/messaging/{pushId}')

.onCreate(event => {

//child is added. Do something!

});