In Firebase is there a way to get the number of children of a node without loading all the node data

Question

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

User · Answer

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!).

User · Answer

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   gt           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

User · Answer

The code snippet you gave does indeed load the entire set of data and then counts it client-side  which can be very slow for large amounts of data   Firebase doesn t currently have a way to count children without loading data  but we do plan to add it   For now  one solution would be to maintain a counter of the number of children and update it every time you add a new child  You could use a transaction to count items  like in this code tracking upvodes   var upvotesRef   new Firebase  https   docs-examples firebaseio com android saving-data fireblog posts -JRHTHaIs-jNPLXOQivY upvotes    upvotesRef transaction function  current value      return  current value    0    1        For more info  see https   www firebase com docs transactions html  UPDATE  Firebase recently released Cloud Functions  With Cloud Functions  you don t need to create your own Server  You can simply write JavaScript functions and upload it to Firebase  Firebase will be responsible for triggering functions whenever an event occurs   If you want to count upvotes for example  you should create a structure similar to this one        posts           -JRHTHaIs-jNPLXOQivY             upvotes count  5         upvotes             userX    true         userY    true         userZ    true                                And then write a javascript function to increase the upvotes count when there is a new write to the upvotes node   const functions   require  firebase-functions    const admin   require  firebase-admin    admin initializeApp functions config   firebase    exports countlikes   functions database ref   posts  postid upvotes   onWrite event   gt      return event data ref parent child  upvotes count   set event data numChildren           You can read the Documentation to know how to Get Started with Cloud Functions   Also  another example of counting posts is here  https   github com firebase functions-samples blob master child-count functions index js  Update January 2018  The firebase docs have changed so instead of event we now have change and context   The given example throws an error complaining that event data is undefined  This pattern seems to work better   exports countPrescriptions   functions database ref   prescriptions   onWrite  change  context    gt        const data   change after val        const count   Object keys data  length      return change after ref child   count   set count

User · Answer

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

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

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