Firebase Cloud Functions -how to get values from different child snapshots at the same ref

Question

I have to get different values from different children at the same ref. In Swift I would do:

var sum = 0
var totalPosts = 0
var starRating = 0 // run some calculations on totalPosts + sum

let sneakersRef = Database.database().reference().child("sneakers").child(adidas_ABC) // postId
sneakersRef.observeSingleEvent(of: .value, with: { (snapshot) in

    if !snapshot.exists() { return }

    if snapshot.hasChild("postCount") {
        let childSnapshot = snapshot.childSnapshot(forPath: "postCount")
            if let postCount = childSnapshot.value as? Int {
                
               self.totalPosts = postCount
            }
        }
    }

    if snapshot.hasChild("fans") {

        let childSnapshot = snapshot.childSnapshot(forPath: "fans")

        for child in childSnapshot.children {

            let userId = child as! DataSnapshot

            for snap in userId.children {

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

                let price = dict["price"] as? Int ?? 0

                self.sum += price
            }

            // update ref with value obtained from sum and create/update a starRating
        }
    }
})

How would I do the same thing in Cloud Functions? The code inside the childSnapshot.forEach((child) => { } below is what I need help with.

exports.updateValues = functions.https.onCall((data, context) => {

    const postId = data.postId; // adidas_ABC

    var sum = 0;
    var totalPosts = 0;
    var starRating = 0;

    const sneakersRef = admin.database().ref('sneakers').child(postId);

    sneakersRef.once('value', snapshot => {

        if (snapshot.exists()) {

            const postCount = snapshot.child("postCount").val();

            totalPosts = postCount

            const childSnapshot = snapshot.child("fans").val().userID;

            // the code in this loop is questionable
            childSnapshot.forEach((snap) => {

                const price = snap.child("price").val();

                sum += price
            })

            // run calculations to get starRating

            return sneakersRef.update({ "sum": sum, "starRating": starRating }).then(() => {            
            })
            .catch((error) => {
                console.log('something went wrong: ', error);
            });
        } else {
            console.log('doesn't exist');
        }
    });
});

Here is my db layout:

@sneakers
   @adidas_ABC // same postId for anything dealing with adidas
      @fans // this might not exist in which case postCount would be zero
         @userId_XYZ
            @postId_123
               -condition: "used"
               -price: 100
               -type: shell_top
            @postId_456
               -condition: "new"
               -price: 500
               -type: yeezy
      -fanCount: 1
      -postCount: 2
      -sum: 600
      -starRating: 4 

Answer 1

There are two things to adapt in your code:

1. You need to loop over the user objects and then you need to loop over each post of each user object. See this SO answer for more details.
2. You need to correctly manage the life cycle of your Cloud Function by returning the Promises chain. As you will see here in the doc, it is key to terminate Cloud Functions that perform asynchronous processing by returning a JavaScript promise. In your case we return the Promise chain, which is a Promise, and which is composed of the Promises returned by the asynchronous Firebase methods (once() and update()) and by then(). For that we need to use the "promisified" version of the once() method (i.e. once('value').then((snapshot) => {..})) and not the callback version, which you used in your code.

So the following should do the trick:

exports.updateValues = functions.https.onCall((data, context) => {
    const postId = data.postId; // adidas_ABC

    var sum = 0;
    var totalPosts = 0;
    var starRating = 0;

    const sneakersRef = admin.database().ref('sneakers').child(postId);

    return sneakersRef   // Note the return here!! => We return the Promise chain
        .once('value')
        .then((snapshot) => {

            if (snapshot.exists()) {
                const postCount = snapshot.child('postCount').val();

                totalPosts = postCount;

                const childSnapshot = snapshot.child('fans').val();

                Object.keys(childSnapshot).forEach((user) => {
                    const obj = childSnapshot[user];
                    Object.keys(obj).forEach((e) => {
                        const price = obj[e].price;
                        sum += price;
                    });
                });

                // I let you dealing with the calculations to get starRating

                return sneakersRef.update({ sum: sum, starRating: starRating });
            } else {
                console.log("doesn't exist");
                return null; // Note the return here
            }
        })
        .catch((error) => {
            console.log('something went wrong: ', error);
            return null; // Note the return here
        });

});