How to only get new data without existing data from a Firebase?

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?

Solution 1:

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.

Solution 2:

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)
    })