[database] In Firebase, is there a way to get the number of children of a node without loading all the node data?

You can get the child count via

firebase_node.once('value', function(snapshot) { alert('Count: ' + snapshot.numChildren()); });

But I believe this fetches the entire sub-tree of that node from the server. For huge lists, that seems RAM and latency intensive. Is there a way of getting the count (and/or a list of child names) without fetching the whole thing?

This question is related to database firebase count

The answer is


This is a little late in the game as several others have already answered nicely, but I'll share how I might implement it.

This hinges on the fact that the Firebase REST API offers a shallow=true parameter.

Assume you have a post object and each one can have a number of comments:

{
 "posts": {
  "$postKey": {
   "comments": {
     ...  
   }
  }
 }
}

You obviously don't want to fetch all of the comments, just the number of comments.

Assuming you have the key for a post, you can send a GET request to https://yourapp.firebaseio.com/posts/[the post key]/comments?shallow=true.

This will return an object of key-value pairs, where each key is the key of a comment and its value is true:

{
 "comment1key": true,
 "comment2key": true,
 ...,
 "comment9999key": true
}

The size of this response is much smaller than requesting the equivalent data, and now you can calculate the number of keys in the response to find your value (e.g. commentCount = Object.keys(result).length).

This may not completely solve your problem, as you are still calculating the number of keys returned, and you can't necessarily subscribe to the value as it changes, but it does greatly reduce the size of the returned data without requiring any changes to your schema.


write a cloud function to and update the node count.

// below function to get the given node count.
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

exports.userscount = functions.database.ref('/users/')
    .onWrite(event => {

      console.log('users number : ', event.data.numChildren());


      return event.data.ref.parent.child('count/users').set(event.data.numChildren());
    }); 

Refer :https://firebase.google.com/docs/functions/database-events

root--| |-users ( this node contains all users list) |
|-count |-userscount : (this node added dynamically by cloud function with the user count)


Save the count as you go - and use validation to enforce it. I hacked this together - for keeping a count of unique votes and counts which keeps coming up!. But this time I have tested my suggestion! (notwithstanding cut/paste errors!).

The 'trick' here is to use the node priority to as the vote count...

The data is:

vote/$issueBeingVotedOn/user/$uniqueIdOfVoter = thisVotesCount, priority=thisVotesCount vote/$issueBeingVotedOn/count = 'user/'+$idOfLastVoter, priority=CountofLastVote

,"vote": {
  ".read" : true
  ,".write" : true
  ,"$issue" : {
    "user" : {
      "$user" : {
        ".validate" : "!data.exists() && 
             newData.val()==data.parent().parent().child('count').getPriority()+1 &&
             newData.val()==newData.GetPriority()" 

user can only vote once && count must be one higher than current count && data value must be same as priority.

      }
    }
    ,"count" : {
      ".validate" : "data.parent().child(newData.val()).val()==newData.getPriority() &&
             newData.getPriority()==data.getPriority()+1 "
    }

count (last voter really) - vote must exist and its count equal newcount, && newcount (priority) can only go up by one.

  }
}

Test script to add 10 votes by different users (for this example, id's faked, should user auth.uid in production). Count down by (i--) 10 to see validation fail.

<script src='https://cdn.firebase.com/v0/firebase.js'></script>
<script>
  window.fb = new Firebase('https:...vote/iss1/');
  window.fb.child('count').once('value', function (dss) {
    votes = dss.getPriority();
    for (var i=1;i<10;i++) vote(dss,i+votes);
  } );

function vote(dss,count)
{
  var user='user/zz' + count; // replace with auth.id or whatever
  window.fb.child(user).setWithPriority(count,count);
  window.fb.child('count').setWithPriority(user,count);
}
</script>

The 'risk' here is that a vote is cast, but the count not updated (haking or script failure). This is why the votes have a unique 'priority' - the script should really start by ensuring that there is no vote with priority higher than the current count, if there is it should complete that transaction before doing its own - get your clients to clean up for you :)

The count needs to be initialised with a priority before you start - forge doesn't let you do this, so a stub script is needed (before the validation is active!).


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