In Firebase, how can I query the most recent 10 child nodes?

Question

I'm using childByAutoId() to generate my children. Each child looks like:

{
    user_id: 1
}

I'd like to get the last 10 most recently added, sorted by time DESC. What's the easiest way to do this?

Answer 1

The answer is that you need to use a bit of reverse logic, and also store a timestamp key:value pair within each node as a negative value. I omitted the user_id: 1 to keep the answer cleaner.

Here's the Firebase structure

"test" : {
    "-KFUR91fso4dEKnm3RIF" : {
      "timestamp" : -1.46081635550362E12
    },
    "-KFUR9YH5QSCTRWEzZLr" : {
      "timestamp" : -1.460816357590991E12
    },
    "-KFURA4H60DbQ1MbrFC1" : {
      "timestamp" : -1.460816359767055E12
    },
    "-KFURAh15i-sWD47RFka" : {
      "timestamp" : -1.460816362311195E12
    },
    "-KFURBHuE7Z5ZvkY9mlS" : {
      "timestamp" : -1.460816364735218E12
    }
  }

and here's how that's written out to Firebase; I just used a IBAction for a button to write out a few nodes:

let testRef = self.myRootRef.childByAppendingPath("test")

let keyRef = testRef.childByAutoId()

let nodeRef = keyRef.childByAppendingPath("timestamp")

let t1 = Timestamp

nodeRef.setValue( 0 - t1) //note the negative value

and the code to read it in

    let ref = self.myRootRef.childByAppendingPath("test")
    ref.queryOrderedByChild("timestamp").queryLimitedToFirst(3).observeEventType(.ChildAdded, withBlock: { snapshot in
        print("The key: \(snapshot.key)") //the key
    })

and I declared a little function to return the current Timestamp

var Timestamp: NSTimeInterval {
    return NSDate().timeIntervalSince1970 * 1000
}

and the output

The key: -KFURBHuE7Z5ZvkY9mlS
The key: -KFURAh15i-sWD47RFka
The key: -KFURA4H60DbQ1MbrFC1

As you can see, they are in reverse order.

Things to note:

1. Writing out your timestamp as negative values
2. When reading in use .queryLimitedToFirst instead of last.

On that note, you can also just read the data as usual and add it to an Array then then sort the array descending. That puts more effort on the client and if you have 10,000 nodes may not be a good solution.

Answer 2

I'm assuming your data actually looks like this:

someDataSet: {
    longUID-1: {
        timeCreated: 9999999999, // (seconds since the javascript epoch)
        user_id: 1
    },
    longUID-2: {
        timeCreated: 1111111111,
        user_id: 2
    },
    longUID-3: {
        timeCreated: 3141592653,
        user_id: 3
    }
}

You could automate that by calling Firebase.push({user_id: ###, timeCreated: ###}) multiple times in a for loop or any other method. Maybe you're adding news stories to a webpage, but you only want your user to see the most current stories--- IDK. But the answer to your question is to use Firebase's ref.orderByChild() and ref.limitToLast().

var ref = new Firebase("<YOUR-FIREBASE-URL>.firebaseio.com/someDataSet"); 
    //the "/someDataSet" comes from the arbitrary name that I used up above

var sortedRef = ref.orderByChild('timeCreated');
    //sort them by timeCreated, ascending

sortedRef.limitToLast(2).on("child_added", function(snapshot){
    var data = snapshot.val();

    console.log(data);

    /* do something else with the data */
});

//The console would look like this

// Object {timeCreated: 9999999999, user_id: 1}
// Object {timeCreated: 3141592653, user_id: 3}

This happened because the program took the child with the greatest timeCreated value first and then the second greatest (value) second... Also note, the longUID means nothing when you sort them by child and neither do the other values (user_id in this case)

Here is the documentation for:

• Firebase .push() method (Sorry, I'm not allowed to post this link- I dont have enough reputation)
• Firebase .orderByChild method
• And also, Firebase .limitToLast method