如何使用Javascript在Firebase数据库中获取children的值

Question

如何使用javascript获取firebase中特定键值对的值？我正在为firebase云消息创建一个功能。我的功能如下：

'use strict'
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
exports.sendNotification = functions.database.ref('/notifications/{receiver_user_id}/{notification_key}').onWrite((event, context)=>{
const receiver_user_id = context.params.receiver_user_id;
const notification_key = context.params.notification_key;
console.log('We have a notification to send to : ', receiver_user_id);
// Grab the current value of what was written to the Realtime Database.
const snapshot = event.after.val();
console.log('Uppercasing', context.params.notification_key, snapshot);
console.log('original value : ', snapshot);
if(!event.after.val()){
console.log('A notification has been deleted: ', notification_key);
return null;
}
const sender_fullname = admin.database().ref(`/notifications/${receiver_user_id}/{notification_key}/notifying_user_fullname`).once('value').toString();
console.log('full name value : ', sender_fullname);
const DeviceToken = admin.database().ref(`/tokens/${receiver_user_id}/device_token`).once('value');
return DeviceToken.then(result=>{
const token_id = result.val();
console.log('token id value : ', token_id);
const payload = {
notification: {
title: sender_fullname.toString(),
body: "You have a new message!",
icon: "default"
}
};
return admin.messaging().sendToDevice(token_id, payload).then(response=>{
console.log('Message has been sent');
});
});
});

现在sender_fullname在控制台日志和发送的通知中生成[object Promise]。我不知道如何得到确切的数值。我的实时数据库中的一个示例条目如下：

original value :  { date_created: '02-21-2020T17:50:32',
my_id: '0ntpUZDGJnUExiaJpR4OdHSNPkL2',
notification_key: '-M0dwVL3w1rKyPYbzUtL',
notification_type: 'liked',
notifying_user: 'OiBmjJ7yAucbKhKNSHtYHsawwhF2',
notifying_user_fullname: 'Captain Proton',
post_key: '-LzSJrOq9Y7hGgoECHRK',
read: 'false' }

有什么方法可以得到"notifying_user_fullname"的确切值吗？如有任何帮助，我们将不胜感激。

Answer 1

要获得sender_fullname的值，您必须按照对DeviceToken所做的方式！

once()方法返回一个用DataSnapshot解析的promise，因此需要使用then()方法来获得DataSnapshot，然后使用val()方法。

因此，以下内容应该可以发挥作用(未经测试(：

exports.sendNotification = functions.database.ref('/notifications/{receiver_user_id}/{notification_key}')
.onWrite((event, context) => {
const receiver_user_id = context.params.receiver_user_id;
const notification_key = context.params.notification_key;
console.log('We have a notification to send to : ', receiver_user_id);
// Grab the current value of what was written to the Realtime Database.
const snapshot = event.after.val();
console.log('Uppercasing', context.params.notification_key, snapshot);
console.log('original value : ', snapshot);
if (!event.after.val()) {
console.log('A notification has been deleted: ', notification_key);
return null;
}
let sender_fullname;
return admin.database().ref(`/notifications/${receiver_user_id}/${notification_key}/notifying_user_fullname`).once('value')
.then(dataSnapshot => {
sender_fullname = dataSnapshot.val();
return admin.database().ref(`/tokens/${receiver_user_id}/device_token`).once('value');
})
.then(dataSnapshot => {
const token_id = dataSnapshot.val();
console.log('token id value : ', token_id);
const payload = {
notification: {
title: sender_fullname,
body: "You have a new message!",
icon: "default"
}
};
return admin.messaging().sendToDevice(token_id, payload)
})
.then(() => {
console.log('Message has been sent');
return null;  // <-- Note the return null here, to indicate to the Cloud Functions platform that the CF is completed
})
.catch(error => {
console.log(error);
return null;
})
});

---

请注意，我们如何将异步方法返回的不同Promise链接起来，以便在云函数中返回Promise，这将向平台指示云函数的工作已经完成。

我建议您观看Firebase系列视频中关于"JavaScript承诺"的3个视频，这些视频解释了这一点的重要性。