How to sort Firebase real-time database by the child node with the highest number of

I am working on a database feature that allows users to 'like' comments on posts.

Is there a way for me to sort the comments based on the number of likes they have?

Although I am aware of using .orderByChild(), my issue lies in not having a separate child for the 'like' count.

Currently, I store user UIDs and timestamps within the original commentID child to handle likes.

Would it be feasible to retrieve comments based on the child with the highest number of sub-children?

The structure of my database is as follows:

{
 "users" : {
  "$uid" : {
   "posts" : {
    "$postID" : {
     "postText" : "blah blah blah",
      "comments" : {
       "$commentID" : {
        "commentText" : "blah blah blah",
         "likes" : {
          "UID01" : "1622035190516",
          "UID02" : "1622036141955",
          "UID03" : "1622036145134",
         }
       },
       "$commentID" : {
        "commentText" : "blah 2 blah 2 blah 2",
         "likes" : {
          "UID01" : "1622036141955",
          "UID02" : "1622036145134",
         }
       }
      }
    }
   }
  }
 }
}
javascriptfirebasefirebase-realtime-database

Answer №1

Is there a way to retrieve comments based on the child with the highest number of children?

Unfortunately, it is not possible to sort data based on dynamic aggregations.

In the realm of NoSQL databases, a common solution is to maintain a counter. A recommended approach is using a Cloud Function, such as Firebase's Cloud Functions, for backend calculations done via Admin SDK. This allows you to secure the database nodes containing the like counts with appropriate security rules to prevent unauthorized modifications.

Specifically, you should:

1/ Initialize the counter to 1 when the first like is recorded. The following onCreate Cloud Function can be used for this purpose:

exports.initializeCounter = functions.database.ref('users/{uid}/posts/{postID}/comments/{commentID}/likes')
    .onCreate((snapshot, context) => {
      return snapshot.ref.parent.child('nbrOfLikes').set(1);
    });

2/ Update the counter whenever a like is added or removed using an onUpdate Cloud Function that calculates the total likes and updates the counter accordingly. The below Cloud Function demonstrates this process:

exports.updateCounter = functions.database.ref('users/{uid}/posts/{postID}/comments/{commentID}/likes')
    .onUpdate((change, context) => {
        const likeObject = change.after.val();
        return change.after.ref.parent.child('nbrOfLikes').set(Object.keys(likeObject).length);
    });

An alternative method would be utilizing the numChildren() method, as discussed in this answer.

Keep in mind the scenario where all likes are deleted, if applicable in your application.

It is worth noting that the two (or three) Cloud Functions can possibly be consolidated into a single onWrite Cloud Function.

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