Retrieving Firebase value using `once` callback with transaction - javascript

I would like to use the once method to retrieve data from a Firebase ref, but am not receiving any data in the snapshot.
Here is the simple data retrieval:
EDIT After further inspection, I found that a transaction was being scheduled shortly after the once value listener. (For simplicity I will place the transaction directly after the listener for this code example.)
dataRef.once('value', function (snapshot) {
snapshot.val(); // returns `null`
});
dataRef.transaction(function (currentValue) {
if (currentValue) {
// make changes
}
return currentValue;
});
There is data at this firebase ref. To verify, I tried using on instead of once. In this case, the callback is invoked twice:
dataRef.on('value', function (snapshot) {
snapshot.val(); // returns `null` first, then called again and returns correct value
});
What is causing this behavior? I would expect the snapshot to have the correct value the first time the callback is invoked. Otherwise, the once method has no useful purpose.

In the Firebase docs for transaction there is a third argument applyLocally.
By default, events are raised each time the transaction update function runs. So if it is run multiple times, you may see intermediate states. You can set this to false to suppress these intermediate states and instead wait until the transaction has completed before events are raised.
Because of this default value, the once value listener is firing after the transaction runs locally the first time, even though no data has been retrieved from the server (passing null as the transaction's currentValue). By passing false as the applyLocally parameter, this local event is suppressed, and the once callback will only be invoked after data has been retrieved from the server:
dataRef.transaction(function (currentValue) {
if (currentValue) {
// make changes
}
return currentValue;
},
function(){},
false);

Related

Is there a way to get the actual timestamp when using firebase.database.ServerValue.TIMESTAMP?

This question is about the javascript client.
I have code that goes something like this:
const localEvents = [];
const fbEvents = firebase.database().ref("myevents");
fbEvents.on("child_added", function(snapshot) {
const e = snapshot.val();
localEvents.push(e);
});
function createEvent(e) {
e.time = firebase.database.ServerValue.TIMESTAMP;
fbEvents.push(e);
}
After calling createEvent({}), it appears that entries in my localEvents list have time values which are not equal to the actual entries in the database (the client guesses the timestamp and calls the child_added handler before it's actually done a roundtrip to the server). Is there any way to avoid this, and/or is there any way to get a callback when the actual value of the time is known?
It's not possible, using only the snapshot in the listener, to determine if the timestamp comes from the server or is guessed locally.
What you can do instead is use the promise returned from fbEvents.push(e) to determine when the write actually succeeds. A resolved promise which means it was definitely written to Firebase. The listener callback you get after that will contain the server's updated value.
(Note that with Firestore it is possible to determine if a document was fully written to the server or not. Just not with Realtime Database.)

Using data returned from Firebase fetch Javascript

Web dev novice here. I'm using Firebase as my backend, and I have a situation where I need to query the DB for one value and use that returned value to match in another query to the same DB.
Here is my code:
function dataLoad() {
var valueThatINeedToUseElsewhere
userReference.on('value',function(users){
var currentUser = firebase.auth().currentUser.uid
users.forEach(function(user){
if(user.key === currentUser) {
valueThatINeedToUseElsewhere = user.val().userName
console.log(valueThatINeedToUseElsewhere)
}
})
})
console.log(valueThatINeedToUseElsewhere)
}
The console.log inside the IF condition logs the correct value. However, the last console.log above does not log anything, despite the variable name being declared outside the 'on' function. Why is this happening? And how can I actually use the data outside the 'on' function? I need to use it to perform a similar IF condition in another data retrieval operation.
Thanks!
You're likely trying to do something like this now:
var valueThatINeedToUseElsewhere;
dataLoad();
doSomethingWith(valueThatINeedToUseElsewhere);
Unfortunately this doesn't work, since (as Doug says) the data is loaded from Firebase asynchronously. By the time you're trying to use valueThatINeedToUseElsewhere, the value hasn't been loaded yet.
Why this doesn't work, the one-three-two test
It's easiest to see this if you place a few log statements like this:
console.log("Before starting to load value")
userReference.on('value',function(users){
console.log("Loaded value")
})
console.log("After starting to load value")
When you run this code, it prints:
Before starting to load value
After starting to load value
Loaded value
That is probably not the order that you expected. But it perfectly explains why you can't use valueThatINeedToUseElsewhere when you try: it hasn't been loaded yet.
Reframing the problem
The best way I've found for dealing with this situation is to reframe the problem. Your current code is written with the logic of "first we load the value, then we do something with the value". For asynchronous loading it's better to frame it as "first start loading the data. once the data has started loading, do something with it".
In code that look like:
function dataLoad() {
userReference.once('value',function(users){
var currentUser = firebase.auth().currentUser.uid
users.forEach(function(user){
if(user.key === currentUser) {
var valueThatINeedToUseElsewhere = user.val().userName
doSomethingWith(valueThatINeedToUseElsewhere)
}
})
})
}
As you'll see, we've moved the code that needs the data into the callback that fires when the data is available. That way you can be guaranteed the data is available when you call doSomethingWith. I also changed the code to use once, which (as Doug said) is better for your use-case.
There are two more steps to take, both to improve the flexibility and performance of your code.
Passing in a function that is called once the data is loaded
First off: with this last update, the call to doSomethingWith is hard-coded, which means that dataLoad() and doSomethingWith are closely tied together. It's often better to keep them more separate. To do that, we can pass in a so-called callback function into dataLoad that it then calls when it has loaded the data. This is very similar to what once() does already. Let's see that in practice:
function dataLoad(callback) {
userReference.once('value',function(users){
var currentUser = firebase.auth().currentUser.uid
users.forEach(function(user){
if(user.key === currentUser) {
var valueThatINeedToUseElsewhere = user.val().userName
callback(valueThatINeedToUseElsewhere)
}
})
})
}
Not too different. But now you can invoke dataLoad like this:
dataLoad(doSomethingWith);
And with that doSomethingWith will be called with the data, once the data has been loaded.
Loading the data by its key
The final change is an optimization. Your current code is loading all users, while you only need one and know their key. It's much more efficient to only load that specific user, with:
function dataLoad(callback) {
var currentUser = firebase.auth().currentUser.uid
userReference.child(currentUser).once('value',function(users){
var valueThatINeedToUseElsewhere = user.val().userName
callback(valueThatINeedToUseElsewhere)
})
}
on() is asychronous and returns immediately, which means your console log is going to show an undefined value. The callback you pass to on() is only going to run when results are available, and there's no guarantee how quickly that will happen. You should only use a value from an asynchronous call after it finishes - don't try to make your code block until some async call is finished.
Also consider using once() instead of on() if you only need the value at a location a single time. You can use its returned promise to chain some additional work after it loads the data you want. It is also asynchronous (as is all other methods that return a promise).
To learn more about why Firebase APIs are asynchronous, and what that means for your app, read this blog.

How to stop reading child_added with Firebase cloud functions?

I try to get all 10 records using this:
exports.checkchanges = functions.database.ref('school/{class}').onCreate(snap => {
const class=snap.params.class;
var ref = admin.database().ref('/students')
return ref.orderByChild(class).startAt('-').on("child_added", function(snapshot) {
const age=snapshot.child("age");
// do the thing
})
)}
The problem is that after I get the 10 records I need correctly, even after few days when a new record is added meeting those terms, this function is still invoked.
When I change on("child_added to once("child_added I get only 1 record instead of 10. And when I change on("child_added to on("value I get null on this:
const age=snapshot.child("age");
So how can I prevent the function from being invoked for future changes?
When you implement database interactions in Cloud Functions, it is important to have a deterministic end condition. Otherwise the Cloud Functions environment doesn't know when your code is done, and it may either kill it too soon, or keep it running (and thus billing you) longer than is necessary.
The problem with your code is that you attach a listener with on and then never remove it. In addition (since on() doesn't return a promise), Cloud Functions doesn't know that you're done. The result is that your on() listener may live indefinitely.
That's why in most Cloud Functions that use the Realtime Database, you'll see them using once(). To get all children with a once(), we'll listen for the value event:
exports.checkchanges = functions.database.ref('school/{class}').onCreate(snap => {
const class=snap.params.class;
var ref = admin.database().ref('/students')
return ref.orderByChild(class).startAt('-').limitToFirst(10).once("value", function(snapshot) {
snapshot.forEach(function(child) {
const age=child.child("age");
// do the thing
});
})
)}
I added a limitToFirst(10), since you indicated that you only need 10 children.

Child_added subscription seems to download entire dataset

I have a large data set (~100k entries), that is being subscribed to using the 'child_added' event. Using node 7 and firebase 3.6.1, doing this seems to download the entire 100k entries before a single child_added event is fired.
Memory consumption grows significantly for a few dozen seconds, and then all child_added events are fired swiftly after each other.
This is slow:
require('firebase').
initializeApp({databaseURL: 'https://someproject.firebaseio.com'}).
database().ref('data').
on('child_added', (snap) => console.log(snap.key));
Limiting is still fast (few seconds delay):
require('firebase').
initializeApp({databaseURL: 'https://someproject.firebaseio.com'}).
database().ref('data').limitToFirst(10).
on('child_added', (snap) => console.log(snap.key));
Given the streaming nature of Firebase, I assume it is not intended behaviour for child_added subscriptions to download the entire data set to the client before anything is done.
Am I doing something wrong, or is this a bug?
Although that in the child_added section extracted from firebase documentation it says:
The child_added event is typically used when retrieving a list of items from the database. Unlike value which returns the entire contents of the location, child_added is triggered once for each existing child and then again every time a new child is added to the specified path. The event callback is passed a snapshot containing the new child's data. For ordering purposes, it is also passed a second argument containing the key of the previous child.
At the first lines in that page, we can found this:
Data stored in a Firebase Realtime Database is retrieved by attaching an asynchronous listener to a database reference. The listener will be triggered once for the initial state of the data and again anytime the data changes.
Seems to be its normal behaviour. It first retrieves all the data.
I am in the same situation, waiting nearly 40 seconds for the first child to fire. The only solution I could come up with was to get the keys using Firebase rest API and shallow query parameter, then loop over each key and call Firebase. Here is basically what I did.
`
console.log('start', Date.now());
fetch('https://[firebase_app].firebaseio.com/[your_path].json?shallow=true')
.then((response) => {
return response.json();
}).then(function(j) {
Object.keys(j).forEach(function (key) {
console.log(key, 'start', Date.now());
firebase_reference.child(key).on("child_added", function (snapshot) {
console.log(key, Date.now());
//now you have the first response without waiting for everything.
});
});
});`
I know this doesn't answer your question about child_added functionality, but it does what you would expect to happen with child_added. I am going to submit a feature request to Firebase and link this SO question.

Firebase Web SDK - Transaction causing on('child_added', func.. to be called

See jsbin.com/ceyiqi/edit?html,console,output for a verifiable example.
I have a reference listening to a database point
jobs/<key>/list
where <key> is the teams unique number
Under this entry point is a list of jobs
I have a listener on this point with
this.jobsRef.orderByChild('archived')
.equalTo(false)
.on('child_added', function(data) {
I also have a method that does the following transaction:
ref.transaction(function(post) {
// correct the counter
if(post)
{
// console.log(post);
if(active)
{
// if toggeling on
}
else
{
// if toggeling off
}
}
return post;
})
When invoking the transaction the child_added is also invoked again, giving me duplicate jobs.
Is this expected behavior?
Should I simply check to see if the item has been got before and add to the array accordingly?
Or am I doing something wrong?
Thanks in advance for your time
You're hitting an interesting edge case in how the Firebase client handles transactions. Your transaction function is running twice, first on null and then on the actual data. It's apparent to see if you listen for value events on /jobs, since then you'll see:
initial value
null (when transaction starts and runs on null)
initial value again (when transaction runs again on real data)
The null value in step 2 is the client's initial guess for the current value. Since the client doesn't have cached data for /jobs (it ignores the cached data for /jobs/list), it guesses null. It is clearly wrong. But unfortunately has been like this for a long time, so it's unlikely to change in the current major version of the SDK.
And because of the null in step 2, you'll get child_removed events (which you're not handling right now) and then in step 3 you'll get child_added events to re-add them.
If you handled the child_removed events, you're items wouldn't end up duplicated, but they would still disappear / reappear, which probably isn't desirable. Another workaround in the current setup is to explicitly tell the transaction to not run with the local estimate, which you can do by passing in false as the third parameter:
function transact() {
var path = 'jobs/';
var ref = firebase.database().ref(path);
ref.transaction(function(post) {
return post;
}, function(error, committed, snapshot) {
if (error) {
console.log('Transaction failed abnormally!', error);
} else if (!committed) {
console.log('Transaction aborted.');
} else {
console.log('Transaction completed.');
}
}, false);
}
I'm sorry I don't have a better solution. But as I said: it's very unlikely that we'll be able to change this behavior in the current generation of SDKs.

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