How to get sum of firestore document values using fold? Flutter

Question

body: StreamBuilder<QuerySnapshot>(
      stream: record,
      builder:
          (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
          ..........

          ..........
        final totalIncome =
            snapshot.data?.docs.fold<double>(0, (previousValue, element) {
          return previousValue + (element['incomeAmount'] ?? 0.0);
        }) as double;

     ...........
     ...........

This works if a field exists. If there is no field in firestore this gives an error field does not exist within the DocumentSnapshotPlatform

How can I make it work something like

element?['incomeAmount']

or if there is no field return 0?

Answer 1

You can make a function to get the totalAmount . It can be look like this

 int totalCost = 0;

  void getTotal() {
    FirebaseFirestore.instance.
        .collection('your_collection_name')
        .snapshots()
        .listen(event) {
      totalCost = (event.docs.fold<int>(
        0,(p, c) => prev + (Model.fromJson(c.data() as Map<String, dynamic>).amount),
      );
    });
  }

Then in the didChangeDependencies you can write this code,

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    if (mounted) {
      getTotal();
    }
  }

Finally, you can show the value of totalCost wherever you want.

 Text(totalCost.toString()),

You can see this GIF to check if it is correct or wrong