无法计算 Firebase 云触发器 function 中的新旧值差异

Question

This cloud function scans new entries in one collection and adjusts data in the other.

Information from my orders collection is being sent to cars collection with their matching reference fields. However, the issue appears on the second function that updates data, when adjusting shipping costs it reduces totalSales with the number it is updated with, rather than the difference between them, meaning that shippingCost is substracting more than it should be.

I have tried to implement change.before.data but without any success.

The code itself is:

  exports.useWildcard = functions.firestore
        .document('orders/{docId}')
        .onCreate((change, context) => 
        {
            var tasks = new Promise((resolve, reject) => {
                admin.firestore().collectionGroup('cars').where("carRef","==",change.data().sku).get().then(documents => {
                    docID = '';
                    documents.forEach((doc) => {                  
                        docID = doc.id;
                        totalSales = 0;
                        orderPrice = 0;
                        shippingCost = 0;
                        if(typeof doc.data().totalSales != "undefined")
                            totalSales = parseFloat(doc.data().totalSales);                        
                        if(typeof change.data().orderPrice != "undefined")
                            orderPrice = parseFloat(change.data().orderPrice);                        
                        if(typeof change.data().shippingCost != "undefined")
                            shippingCost = parseFloat(change.data().shippingCost);
                        newTotalSales = totalSales-shippingCost;
                        admin.firestore().collection('cars').doc(docID).update({"totalSales":newTotalSales}).then(documents => {
                            resolve();
                        });
                    });
                    if(docID=='') resolve();
                });
            });
            return tasks;                       
       }
    );

The issue is that the totalSales subtract shipping costs as instructed in the code, but some orders may have further shipping cost adjustments, creating an issue of new value being deducted in addition to previous one. In this case, I would like to either deduct or add shippingCost difference to totalSales field.

Answer 1

I think you should use onWrite Event instead of onCreate event on your orders collecion which would give you access to change.before.data(); and change.after.data(); .

• change.before.data(); would help you to access previous value instead of new value being subtracted.

Check this if you need more info.