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I have a node in Firebase getting continually updated with information from a logfile. The node is lines/ and each child of lines/ is from a post() so it has a unique ID.

When a client first loads, I want to be able to grab the last X number of entries. I expect I'll do this with once(). From then on, however, I want to use an on() with child_added so that I get all new data. However, child_added gets all data stored in the Firebase and, after the initial setup, only want the new stuff.

I see that I can add a limitToLast() on the on(), but, if I say limitToLast(1) and a flood of entries come in, will my app still get all the new entries? Is there some other way to do this?

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  • If my answer solved your problem, would you mind marking it as the solution? If not let me know what else is needed. Commented Nov 26, 2015 at 21:46

4 Answers 4

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You need to include a timestamp property and run a query.

// Get the current timestamp var now = new Date().getTime(); // Create a query that orders by the timestamp var query = ref.orderByChild('timestamp').startAt(now); // Listen for the new children added from that point in time query.on('child_added', function (snap) { console.log(snap.val() }); // When you add this new item it will fire off the query above ref.push({ title: "hello", timestamp: Firebase.ServerValue.TIMESTAMP });

The Firebase SDK has methods for ordering, orderByChild() and methods for creating a range startAt(). When you combine the two you can limit what comes back from Firebase.

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The problem in your solution is the new Date().getTime() may not get the right timestamp if the client device is not be synchronized.
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I think there is a problem in @David East's solution. He is using the local timestamp which may cause problem if the time is not accurate in client device. Here is my suggested solution (iOS Swift):

  • Using observeSingleEvent to get the complete data set
  • Then returned it in reversed order by reversed()
  • Get the last timestamp by for example data[0].timestamp

  • Using queryStarting for timestamp

     self._dbref.queryOrdered(byChild: "timestamp").queryStarting(atValue: timestamp+1) .observe(.childAdded, with: { snapshot in print(snapshot.value) })
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IDK if this is actual for web, but for android and ios this is super actual
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You have the right idea. child_added should be called only for the new nodes. Without source code it's hard to tell why you get all the data in your child_added event.

You can check the chat demo app to see how they load new chat messages. The use case sounds similar.

https://github.com/firebase/firechat/blob/master/src/js/firechat.js#L347

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The child_added event fires off for all data and then the new nodes. So he'll need to use a combination of a timestamp and a query to get only the latest nodes after a point.
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Here's temporary but quick solution:

// define a boolean var bool = false; // fetch the last child nodes from firebase database ref.limitToLast(1).on("child_added", function(snap) { if (bool) { // all the existing child nodes are restricted to enter this area doSomething(snap.val()) } else { // set the boolean true to doSomething with newly added child nodes bool = true; } });

Disadvantage: It will load all the child nodes.

Advantage: It will not process existing child nodes but just the newly added child nodes.

limitToLast(1) will do the work.

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