How to get all documents in sub-collections to RecyclerView in Firebase Firestore

In my Firestore database, I have a 'categories' collection with multiple documents which in turn have 'products' sub-collections with respective documents.

I'm trying to get all documents in all sub-collections and populate into a Recylerview but I have failed.

Here is the code I have;

 private void populateAllProducts() {
        Log.d(TAG, "populateAllProducts called: ");
          database
                .collection(Globals.CATEGORIES)
                .get()
                .addOnCompleteListener(task -> {
                    if (task.isSuccessful()) {
                            for (QueryDocumentSnapshot document : Objects.requireNonNull(task.getResult())) {
                                Category category = document.toObject(Category.class);
                                Log.d(TAG, "Category: " + category.getName());
                                    database
                                        .collection(Globals.CATEGORIES)
                                        .document(category.getUuid())
                                        .collection(Globals.PRODUCTS)
                                        .get()
                                        .addOnCompleteListener(task1 -> {
                                            if (task.isSuccessful()) {
                                                    for (QueryDocumentSnapshot snapshot : Objects.requireNonNull(task1.getResult())) {
                                                        Product product = snapshot.toObject(Product.class);
                                                        Log.d(TAG, "All products: " + product.getName());
                                                    }
                                                }
                                        })
                                        .addOnFailureListener(e -> Log.e(TAG, "Error: ", e));
                            }
                    }
                })
                .addOnFailureListener(e -> {
                    vars.verityApp.crashlytics.log("Error while fetching categories");
                    Log.e(TAG, "Error occured: ", e);
                    vars.verityApp.crashlytics.recordException(e);
                });
    }

But I receive this Fatal Exception;

2020-08-11 16:27:16.358 26161-26161/com.verityfoods E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.verityfoods, PID: 26161
    java.lang.NullPointerException: Provided document path must not be null.
        at com.google.firebase.firestore.util.Preconditions.checkNotNull(Preconditions.java:147)
        at com.google.firebase.firestore.CollectionReference.document(CollectionReference.java:103)
        at com.verityfoods.ui.bottomviews.shop.ShopFragment.lambda$populateCategories$2$ShopFragment(ShopFragment.java:87)
        at com.verityfoods.ui.bottomviews.shop.-$$Lambda$ShopFragment$dIuQdpUfodkE_zdMSo4L2Rr4DL0.onComplete(Unknown Source:2)
        at com.google.android.gms.tasks.zzj.run(Unknown Source:4)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

I will appreciate your help!

You are getting the following error:

> java.lang.NullPointerException: Provided document path must not be null.

Because you are passing a null value in your reference. Most likely category.getUuid() returns null, hence that error. Seeing your getUuid() getter, I'm can guess that the name of that property is not uuid in the database, so this is most likely the reason.

> I'm trying to get all documents in all sub-collections and populate into a Recylerview

To get all the data in all products subcollections, you should use a Firestore collection group query and not regular CollectionReference object. In code, this query should look like this:

db.collectionGroup("products").get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
@Override
public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
//Iterate to get the products out of the queryDocumentSnapshots object
}
});

You are checking if the FIRST task it's done inside of the second task.

private void populateAllProducts() {
Log.d(TAG, "populateAllProducts called: ");
database
.collection(Globals.CATEGORIES)
.get()
.addOnCompleteListener(task -> {
if (task.isSuccessful()) {
for (QueryDocumentSnapshot document : Objects.requireNonNull(task.getResult())) {
Category category = document.toObject(Category.class);
Log.d(TAG, "Category: " + category.getName());
database
.collection(Globals.CATEGORIES)
.document(category.getUuid())
.collection(Globals.PRODUCTS)
.get()
.addOnCompleteListener(task1 -> {
if (task.isSuccessful()) { //Here should be task1, not task
for (QueryDocumentSnapshot snapshot : Objects.requireNonNull(task1.getResult())) {
Product product = snapshot.toObject(Product.class);
Log.d(TAG, "All products: " + product.getName());
}
}
})
.addOnFailureListener(e -> Log.e(TAG, "Error: ", e));
}
}
})
.addOnFailureListener(e -> {
vars.verityApp.crashlytics.log("Error while fetching categories");
Log.e(TAG, "Error occured: ", e);
vars.verityApp.crashlytics.recordException(e);
});
}

Maybe task1 isn't complete yet when that code reaches and because task it is actually complete, it fires anyway