Firebase功能：无法读取未定义的“ user_id”

Question

我正在尝试通过移动应用实现将推送通知从一台设备发送到另一台设备的功能。通过参考视频教程。这是我当前的javascript代码：

'use-strict'

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
exports.sendNotification = functions.firestore.document("users/{user_id}/notification/{notification_id}").onWrite(event=> {

const user_id = event.params.user_id;
const notification_id = event.params.notification_id;
return admin.firestore().collection("users").doc(notification_id).get().then(queryResult=> {

        const from_user_id = queryResult.data();
        const from_data = admin.firestore().collection("users").doc(from_user_id).get();
        const to_data = admin.firestore().collection("users").doc(user_id).get();
        return Promise.all([from_data, to_data]).the(results =>{
            const from_name = results[0].data().name;
            const to_name = results[1].data().name;
            console.log("from :"+ from_name+" To "+ to_name);
        });


    });
});

这显示以下错误

TypeError: Cannot read property 'user_id' of undefined at exports.sendNotification.functions.firestore.document.onWrite.event (/user_code/index.js:9:30)
at cloudFunctionNewSignature (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:105:23)
    at cloudFunction (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:135:20)
    at /var/tmp/worker/worker.js:733:24
    at process._tickDomainCallback (internal/process/next_tick.js:135:7 

请帮助我。