How to access a value of a map inside list in firebase flutter

Question

I have been trying to get the value merchant_ID from a map inside a List, but I couldn't. please how to query this in flutter?

List<Order?> fetchOrder(QuerySnapshot querySnapshot) {
    
    return querySnapshot.docs.map((order) {
        return Order.fromFirebase(order);
    }).toList();

  }

  Stream<List<Order?>> get Orders {
    return collectionOrd.where("products.merchant",isEqualTo: userID).snapshots().map(fetchOrder);
  }