How to synchronously work with getx controllers, Getx

Question

I am having a scenario , where i have to get Order details from user details i.e UserModel has a attribute of orders , which consists of array of orders,

But user details i.e UserModel will be filled from firebase.

So how to get order details from UserModel , after the UserModel is actually filled from the firebase.

By far my code:

UtilityController.dart

class UtilityController extends GetxController {
  static UtilityController instance = Get.find();
  Rx<UserModel> userModel = UserModel().obs;

  @override
  void onReady() {
    super.onReady();
    userModel.bindStream(listenToUser()); <-- I am using this to actaully fill the user details
  }

  Stream<UserModel> listenToUser() {
    return FirebaseFirestore.instance
        .collection("users")
        .doc(FirebaseAuth.instance.currentUser?.uid)
        .snapshots()
        .map((snapshot) => UserModel.fromSnapshot(snapshot));
  }

OrderController.dart

class OrderController extends GetxController {
  static OrderController instance = Get.find();
  RxList<OrderModel> orders = RxList<OrderModel>([]);

  @override
  void onReady() {
    super.onReady();
    orders.bindStream(getAllOrders());
    ever(utilityController.userModel, function); --> I am using this , but i know this is not the correct way
  }

  function(UserModel userModel) {
    getAllOrders();
  }

  Stream<List<OrderModel>> getAllOrders() {
    return FirebaseFirestore.instance
        .collection("orders")
        .where(FieldPath.documentId, whereIn: utilityController.userModel.value.orders)
        .snapshots()
        .map((query) => query.docs.map((item) => OrderModel.fromMap(item.data(), item.id)).toList());
  }
}

The utilityController.userModel.value.orders is null !!! it's not yet loaded, so all the orders are fetched :( And even if the orders are changed... But new orders are not fetched from the orders collection

How to get over this?