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 mos

Answer 1

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(".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