如何使用 Firebase 云功能更新 Firestore 文档的值

Question

I have firestore database and I would like to trigger a cloud functions when there is a change in a field of a firestore document. I would like the cloud function to look at what has changed and use the new data to update another filed of the firestore document.

Example

Lets assume the document in the picture has changed and has the new values as in the picture below

Now I would like to update the value of accuracy to successes / attempts , ie I would like accuracy to be 6/11 = 0.54

What should I write in the function? Here is what I have so far

exports.calculateAccuracy = functions.firestore.document('/users/{userId}/wordScores')
    .onUpdate((change, context) => {

      //what to write here?

    });

Extra question: how many reads/writes I am going to consume to update the accuracy?

Thanks!!!

Answer 1

The following should do the trick. Notice the path in the Cloud Firestore trigger ( users/{userId}/wordScores/{scoreDocId} ): it points to a Document, not to a Collection. More info in the documentation .

exports.calculateAccuracy = functions.firestore
    .document('users/{userId}/wordScores/{scoreDocId}')
    .onUpdate((change, context) => {

        const newValue = change.after.data();
        const previousValue = change.before.data();

        if ((newValue.attempts !== previousValue.attempts) || (newValue.successes !== previousValue.successes)) {
            return change.after.ref.update({
                accuracy: newValue.successes / newValue.attempts
            });

        } else {
            return null;
        }

    });