How can I loop over the items in the cart, 2) Add each item to the new location, 3) Delete the item from the old location

问题

The items are being displayed using Recycler firestore Adapter. I want to have the items in the recyclerview view to be added in another database and the initial database cleared by a click of a button. I tried this but I got the error that com.google.firebase.firestore.QuerySnapshot cannot be cast to java.util.List

public void placeOrder() {

    FirebaseFirestore rootRef = FirebaseFirestore.getInstance();
    Query query = rootRef
            .collection(String.valueOf(new 
  StringBuffer("/").append(user.getUid()).append("cart")));

    Task task = query.get();

    task.addOnSuccessListener(new OnSuccessListener<List<Object>>() {

        @Override
        public void onSuccess(List<Object> list) {


            CollectionReference notebookRef;
            mAuth = FirebaseAuth.getInstance();
            user = mAuth.getCurrentUser();
            if (user != null) {
                notebookRef = db.collection(String.valueOf(new 
        StringBuffer("/").append(user.getUid()).append("order")));



                String name = String.valueOf(list.get(1));
                String delivery = String.valueOf(list.get(2));
                String amount = String.valueOf(list.get(3));

                OrderItem orderItem = new OrderItem(name, amount, 
                delivery);
                notebookRef.add(orderItem);
                Toast.makeText(context, "Items Added", 
                     Toast.LENGTH_SHORT).show();
            }
           }
        });
       }[![This is my Database][1]][1]

回答1:

The Query.get() method returns a Task<QuerySnapshot>. Since you declared a Task<List> in your code, this results in the error that you get: com.google.firebase.firestore.QuerySnapshot cannot be cast to java.util.List.

Please study the documentation on reading multiple documents from a collection, as it contains the precise code snippet you need.

---

One thing I'm not completely sure of though it whether you are getting an entire collection or a single document from a collection. You initialize query like this:

Query query = rootRef.collection(
  String.valueOf(new StringBuffer("/").append(user.getUid()).append("cart")));

As far as I can see this leads to a path of "/uidOfUser/cart", which would point to the cart document in a collection named after the current user. If that is indeed your database structure, you'll need to follow the example from the documentation on getting a single document, which returns a Task<DocumentSnapshot>.

来源：https://stackoverflow.com/questions/56320635/how-can-i-loop-over-the-items-in-the-cart-2-add-each-item-to-the-new-location