Firebase -how to run a calculation using a Cloud Function

Question

I have a food app with a star system. Every time a user selects a rating/star I send their rating to the database and then run a firebase transaction to update the total amount of users who left a rating.

let theirRating: Double = // whatEverStarRatingTheySelected
let dict = [currentUserId: theirRating]

let reviewsRef = Database.database().reference().child("reviews").child(postId)
reviewsRef.updateChildValues(dict) { (error, ref) in

    let postsRef = Database.database().reference().child("posts").child(postId).child("totalUsersCount")
    postsRef.runTransactionBlock({ (mutableData: MutableData) -> TransactionResult in
        // ...
    })
}

When displaying the actual rating I pull the the post which has the totalUsersCount as a property, I pull all of the users actual ratings, add them all together, and then feed both numbers into an algo to spit out the actual rating. I do all of this on the client. How can I do the same thing "I pull all of the users actual rating, add them all together" with a Cloud Function, that is similar to this answer?

Database.database().reference().child("posts").child(postId).observeSingleEvent(of: .value, with: { (snapshot) in
 
     guard let dict = snapshot.value as? [String:Any] else { return }

     let post = Post(dict: dict)
 
     let totalUsers = post.totalUsersCount

     Database.database().reference().child("reviews").child(postId).observeSingleEvent(of: .value, with: { (snapshot) in
      
          var ratingSum = 0.0
      
          // *** how can I do this part using a Cloud Function so that way I can just read this number as a property on the post instead of doing all of this on the client ***
          for review in (snapshot.children.allObjects as! [DataSnapshot]) {
              guard let rating = review.value as? Double else { continue }
              ratingSum += rating
          }
      
          let starRatingToDisplay = self.algoToComputeStarRating(totalUsersCount: totalUsersCount, ratingsSum: ratingSum)
     })
 })

This isn't a question about the algoToComputeStarRating, that I can do on the client. I want to add all of the user's ratings together using a Cloud Function, then just add that result as a property to the post. That way when I pull the post all I have to do is:

Database.database().reference().child("posts").child(postId).observeSingleEvent(of: .value, with: { (snapshot) in

    guard let dict = snapshot.value as? [String:Any] else { return }

    let post = Post(dict: dict)
         
    let totalUsersCount = post.totalUsersCount
    let ratingsSum = post.ratingsSum
    
    let starRatingToDisplay = self.algoToComputeStarRating(totalUsersCount: totalUsersCount, ratingsSum: ratingSum)
})

Database structure:

@posts
   @postId
      -postId: "..."
      -userId: "..."
      -totalUsersCount: 22
      -ratingsSum: 75 // this should be the result from the cloud function

@reviews
    @postId
       -uid_1: theirRating
       -uid_2: theirRating
       // ...
       -uid_22: theirRating

This is what I tried so far:

exports.calculateTotalRating = functions.https.onRequest((data, response) => {

    const postId = data.postId;
    
    const totalUsersCtRef = admin.database().ref('/posts/' + postId + '/' + 'totalUsersCt');
    const postsRef = admin.database().ref('/posts/' + postId);

    admin.database().ref('reviews').child(postId).once('value', snapshot => {

        if (snapshot.exists()) {

            var ratingsSum = 0.0;

            snapshot.forEach(function(child) {

                ratingsSum += child().val()
            })
            .then(() => { 
            
                return postsRef.set({ "ratingsSum": ratingsSum})          
            })
            .then(() => { 
            
                return totalUsersCtRef.set(admin.database.ServerValue.increment(1));                
            }) 
            .catch((error) => {
                console.log('ERROR - calculateTotalRating() Failed: ', error);
            });
        }
    });
});

Answer 1

I got it to work using this answer:

exports.calculateTotalRating = functions.https.onRequest((data, response) => {

    const postId = data.postId;
    
    const postsRef = admin.database().ref('/posts/' + postId);
    const totalUsersCtRef = admin.database().ref('/posts/' + postId + '/' + 'totalUsersCt');

    var ratingsSum = 0.0;

    admin.database().ref('reviews').child(postId).once('value', snapshot => {

        if (snapshot.exists()) {

            snapshot.forEach((child) => {

                ratingsSum += child().val()
            })
            console.log('ratingsSum: ', ratingsSum);

            return postsRef.update({ "ratingsSum": ratingsSum }).then(() => { 
                return totalUsersCtRef.set(admin.database.ServerValue.increment(1));                
            })
            .catch((error) => {
                console.log('ERROR - calculateTotalRating() Failed: ', error);
            });   
        }
    });
});

Answer 2

I think this should provide you a good start. I have used a trigger so all the updates will happen automatically and your swift listener will immediately get the new ratings. :) Though I am not sure how your let post = Post(dict: dict) is going to parse the [String: Any] dictionary.

//We are adding a event listner that will be triggered when any child under the reviews key is modified. Since the key for updated review can be anything we use wildcards `{autoId}`.
exports.calculateTotalRating = functions.database
  .ref("reviews/{autoId}")
  .onUpdate((snapshot, context) => {


    if (snapshot.exists()) { //For safety check that the snapshot has a value
      const ratingsDict = snapshot.val(); //The updated reviews object you set using `reviewsRef.updateChildValues(dict)`

      const autoId = context.params.autoId; //Getting the wildcard postId so we can use it later 

      //Added due to error in understanding the structure of review object. Can be ignored. 
      //delete ratingsDict("totalUserCount");
      //delete ratingsDict("ratingsSum");

      const totalUsers = Object.keys(ratingsDict).length; //Counting how many reviews are there in the ratingsDict 

      //Sum over all the values in ratingsDict. This approach is supposedly slower, but was oneliner so used it. Feel free to modify this with a more efficient way to sum values once you are more comfortable with cloud functions.
      const ratingsSum = Object.keys(ratingsDict).reduce(
        (sum, key) => sum + parseFloat(ratingsDict[key] || 0),
        0
      );  

      //Saving the computed properties.
      ratingsDict["totalUserCount"] = totalUsers;
      ratingsDict["ratingsSum"] = ratingsSum;

      //Updating posts with new reviews and calculated properties
      return functions.database.ref("posts").child(autoId).set(ratingsDict);
    }
  });

I feel like you are new to this, so you can follow this guide to create cloud functions. In your app's top level directory, just run firebase init functions and selecting your project from Use an existing project. This should create a functions folder for you with an index.js file where you can add this code. Then just run firebase deploy --only functions:calculateTotalRating.