我使用文档的id来标识我想要修改的文档，但是如果我想使用文档内部的值呢？举例来说：

dbUserDoc.collection('shoplist').get().then((querySnapshot) => {
            // ignore: avoid_function_literals_in_foreach_calls
            querySnapshot.docs.forEach((result) {
              final docPro = FirebaseFirestore.instance
                  .collection('products')
                  .doc(result.id); //here i use the document id but i want to use a value inside of that document
              print(result.id);
              docPro.get().then((DocumentSnapshot doc) {
                final data = doc.data() as Map<String, dynamic>;
                final int stockInt =
                    data['stock'] - int.parse(result['quantity']);
                final stock = <String, int>{"stock": stockInt};
                docPro.set(stock, SetOptions(merge: true));
              });
            })
          });

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