Split the child counting into two functions:

1. tracks add/remove of child nodes and increments/decrements the counter
2. one function is triggered when the counter is deleted and it recounts the child nodes

The delta counter in #1 is more efficient than counting all children every time, since it only needs
access to the delta. The recounter function in #2 is equally inefficient as before, but is needed
a lot less.

Author: puf

child-count/README.md

@@ -6,7 +6,12 @@ This template shows how to keep track of the number of elements in a Firebase Da
 
 See file [functions/index.js](functions/index.js) for the code.
 
-This is done by simply updating a `likes_count` attribute on the parent of the list node which is tracked.
+This is done by updating a `likes_count` property on the parent of the list node which is tracked.
+
+This counting is done in two cases:
+
+1. When a like is added or deleted, the `likes_count` is incremented or decremented.
+2. When the `likes_count` is deleted, all likes are recounted.
 
 The dependencies are listed in [functions/package.json](functions/package.json).
 

child-count/functions/index.js

@@ -19,7 +19,28 @@ const functions = require('firebase-functions');
 const admin = require('firebase-admin');
 admin.initializeApp(functions.config().firebase);
 
-// Keeps track of the length of the 'likes' child list in a separate attribute.
-exports.countlikes = functions.database.ref('/posts/{postid}/likes').onWrite(event => {
-  return event.data.ref.parent.child('likes_count').set(event.data.numChildren());
+// Keeps track of the length of the 'likes' child list in a separate property.
+exports.countlikechange = functions.database.ref("/posts/{postid}/likes/{likeid}").onWrite((event) => {
+  var collectionRef = event.data.ref.parent;
+  var countRef = collectionRef.parent.child('likes_count');
+
+  return countRef.transaction(function(current) {
+    if (event.data.exists() && !event.data.previous.exists()) {
+      return (current || 0) + 1;
+    }
+    else if (!event.data.exists() && event.data.previous.exists()) {
+      return (current || 0) - 1;
+    }
+  });
 });
+
+// If the number of likes gets deleted, recount the number of likes
+exports.recountlikes = functions.database.ref("/posts/{postid}/likes_count").onWrite((event) => {
+  if (!event.data.exists()) {
+    var counterRef = event.data.ref;
+    var collectionRef = counterRef.parent.child('likes');
+    return collectionRef.once('value', function(messagesData) {
+      return counterRef.set(messagesData.numChildren());
+    });
+  }
+});