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Question

I'm trying to test out Firebase to allow users to post comments using push. I want to display the data I retrieve with the following;

fbl.child('sell').limit(20).on("value", function(fbdata) { 
  // handle data display here
}

The problem is the data is returned in order of oldest to newest - I want it in reversed order. Can Firebase do this?

Answer 1

Since firebase 2.0.x you can use limitLast() to achieve that:

fbl.child('sell').orderByValue().limitLast(20).on("value", function(fbdataSnapshot) { 
  // fbdataSnapshot is returned in the ascending order
  // you will still need to order these 20 items in
  // in a descending order
}

Here's a link to the announcement: More querying capabilities in Firebase

Answer 2

To augment Frank's answer, it's also possible to grab the most recent records--even if you haven't bothered to order them using priorities--by simply using endAt().limit(x) like this demo:

var fb = new Firebase(URL);

// listen for all changes and update
fb.endAt().limit(100).on('value', update);

// print the output of our array
function update(snap) {
   var list = [];
    snap.forEach(function(ss) {
       var data = ss.val();
       data['.priority'] = ss.getPriority();
       data['.name'] = ss.name();
       list.unshift(data); 
    });
   // print/process the results...
}

Note that this is quite performant even up to perhaps a thousand records (assuming the payloads are small). For more robust usages, Frank's answer is authoritative and much more scalable.

This brute force can also be optimized to work with bigger data or more records by doing things like monitoring child_added/child_removed/child_moved events in lieu of value, and using a debounce to apply DOM updates in bulk instead of individually.

DOM updates, naturally, are a stinker regardless of the approach, once you get into the hundreds of elements, so the debounce approach (or a React.js solution, which is essentially an uber debounce) is a great tool to have.

Answer 3

I had this problem too, I found a very simple solution to this that doesn't involved manipulating the data in anyway. If you are rending the result to the DOM, in a list of some sort. You can use flexbox and setup a class to reverse the elements in their container.

.reverse {
  display: flex;
  flex-direction: column-reverse;
}

Answer 4

Firebase: How to display a thread of items in reverse order with a limit for each request and an indicator for a "load more" button.

• This will get the last 10 items of the list

FBRef.child("childName") .limitToLast(loadMoreLimit) // loadMoreLimit = 10 for example

• This will get the last 10 items. Grab the id of the last record in the list and save for the load more functionality. Next, convert the collection of objects into and an array and do a list.reverse().

• LOAD MORE Functionality: The next call will do two things, it will get the next sequence of list items based on the reference id from the first request and give you an indicator if you need to display the "load more" button.

this.FBRef .child("childName") .endAt(null, lastThreadId) // Get this from the previous step .limitToLast(loadMoreLimit+2)

• You will need to strip the first and last item of this object collection. The first item is the reference to get this list. The last item is an indicator for the show more button.

• I have a bunch of other logic that will keep everything clean. You will need to add this code only for the load more functionality.

    list = snapObjectAsArray;  // The list is an array from snapObject
    lastItemId = key; // get the first key of the list
  
    if (list.length < loadMoreLimit+1) {
      lastItemId = false; 
    }
    if (list.length > loadMoreLimit+1) {
      list.pop();
    }
    if (list.length > loadMoreLimit) {
      list.shift();
    }
    // Return the list.reverse() and lastItemId 
    // If lastItemId is an ID, it will be used for the next reference and a flag to show the "load more" button.
  }
  

Answer 5

There is a better way. You should order by negative server timestamp. How to get negative server timestamp even offline? There is an hidden field which helps. Related snippet from documentation:

var offsetRef = new Firebase("https://<YOUR-FIREBASE-APP>.firebaseio.com/.info/serverTimeOffset");
  offsetRef.on("value", function(snap) {
  var offset = snap.val();
  var estimatedServerTimeMs = new Date().getTime() + offset;
});

Answer 6

To add to Dave Vávra's answer, I use a negative timestamp as my sort_key like so

Setting

const timestamp = new Date().getTime();
const data = {
  name: 'John Doe',
  city: 'New York',
  sort_key: timestamp * -1 // Gets the negative value of the timestamp
}

Getting

const ref = firebase.database().ref('business-images').child(id);
const query = ref.orderByChild('sort_key');
return $firebaseArray(query); // AngularFire function

This fetches all objects from newest to oldest. You can also $indexOn the sortKey to make it run even faster