How to count the number of documents under a collection in Firestore?

Question

I am trying to get CollectionReference count that exists on the Cloud Firestore, I have tried to get it with:

FirebaseFirestore db = FirebaseFirestore.getInstance();
    final CollectionReference postsCollection = db.collection("Posts");

    final TaskCompletionSource<Integer> source = new TaskCompletionSource<>();
    new Thread(new Runnable() {
        @Override
        public void run() {
            int fromWhereToStart = postsCollection.get().getResult().size();
            source.setResult(fromWhereToStart);
        }
    }).start();

    Task<Integer> task = source.getTask();
    task.addOnCompleteListener(new OnCompleteListener<Integer>() {
        @Override
        public void onComplete(@NonNull Task<Integer> task) {
            Log.e("Z_fromWhereToStart", "= " + task.getResult());
        }
    });

But unfortunately, I'm getting:

java.lang.IllegalStateException: Task is not yet complete

Is there is another way to get the count of another way to fix the IllegalStateException?

Solution 1

Edit: July 10th, 2021

Recently, Firebase added a new Extension called Distributed Counter:

Use this extension to add a highly scalable counter service to your app. This is ideal for applications that count viral actions or any very high-velocity action such as views, likes, or shares.

Using this Extension, you can also get over the max limit of one write operation/second.

Here is also an article that you might be interested in:

• How to count the number of documents in a Firestore collection?

---

Becasue there is no getDocumentCount() method as we have in Firebase Realtime database, a getChildrenCount() method, to actually count the number of all documents beneath your Posts collection from your Cloud Firestore, please use the following code:

db.collection("Posts").get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
    @Override
    public void onComplete(@NonNull Task<QuerySnapshot> task) {
        if (task.isSuccessful()) {
            int count = 0;
            for (DocumentSnapshot document : task.getResult()) {
                count++;
            }
            Log.d("TAG", count + "");
        } else {
            Log.d(TAG, "Error getting documents: ", task.getException());
        }
    }
});

or

db.collection("Posts").get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
    @Override
    public void onComplete(@NonNull Task<QuerySnapshot> task) {
        if (task.isSuccessful()) {
            Log.d("TAG", task.getResult().size() + "");
        } else {
            Log.d(TAG, "Error getting documents: ", task.getException());
        }
    }
});

The above examples work well enough for small datasets but it doesn't work if the dataset is larger. But, there are also two more ways in which you can achieve the same thing.

One way would be to use Cloud Functions to update a counter every time you add or delete a document from your Posts collection. This technique works well also for big datasets. But note, in this case, the additions and deletions of documents can only occur at the rate less than or equal to 1 per second, as described in Cloud Firestore Quotas and Limits. That is a single document to read but it shows you the current count almost instantly.

If there is a need for you to exceed this limitation, you need to implement distributed counters as explained in the official documentation of distributed counters.

As a personal hint, don't store this kind of counter in Cloud Firestore, because every time you increase or decrease the counter will cost you a read or a write operation. Host this counter in the Firebase Realtime database almost at no cost.

The second way would be, rather than using Cloud Functions, to use transactions at your client-side, to update the counter at the same time as you add or delete a document. In this way, your counter will also be accurate, because it is updated at the same time. But the most important thing, in this case, is that you'll need to make sure to include this logic anywhere you add or delete a document. You can use in this case Firebase Realtime database as well, at no cost.

In conclusion, use the first code for small datasets, the second use Cloud Functions because is write-time best-effort, and the third use the last option I have explained to you above.

Solution 2

inside onCompelete() check if (task.isSuccessful())before using logs. and also you can iterate over the documentSnapShot like for (DocumentSnapshot document : task.getResult()){ yourCounter++;} and increment counter on each iteration to know how many documents under collection "Posts" are available

Solution 3

I did solve my problem by this code:

db.collection("Posts").get().addOnCompleteListener(new 
 OnCompleteListener<QuerySnapshot>() {
    @Override
    public void onComplete(@NonNull Task<QuerySnapshot> task) {
        if (task.isSuccessful()) {
               count = task.getResult().size();              
        } else {
               Toast.makeTesxt(getContext(),"Error : " + 
                                         e.toString(),Toast.LENGHT_LONG).show;
        }
     }
});

Comments

[Dasser Basyouni]

Thank you, that is working but is there isn't a way to get them without getting the whole data?

[Alex Mamo]

Unfortunatelly, there is not getDocumentCount() method as we have in Firebase Realtime database, getChildrenCout().

[Dasser Basyouni]

thank so much for your help, I think they have to create a method for me then xD

[Alex Mamo]

I'll update my answer with few more specifications in a few minutes.

[Alex Mamo]

Please see my updated answer. Hope you'll have a better understanding regarding counting documents in Cloud Firestore.

[Dasser Basyouni]

sorry for the late reply, thank you so much for your perfect explained answer appreciated, I don't know which answer deserve the accepted sign, is it the first one who answers?

[Alex Mamo]

It's not about who has answered first, it's your call to check as accepted the answer that you think is more useful for you. This will also help future visitors that will read the question/answer.

[Dasser Basyouni]

thank you for explaining to me this point, have a nice day you and Mohammed

[Mohammed Farhan]

@AlexMamo thanks for introducing distributed counters concepts. Cheers!

[Mohammed Farhan]

@AlexMamo you deserve more than one +1

[tccpg288]

This solution is for .get() - what about if you use .addSnapshotListener() for realtime updates?

[Alex Mamo]

@tccpg288 .addSnapshotListener() is used to listen to document changes.

[Al Foиce ѫ]

Code-only answers are not particularly helpful. Please add some descriptions of how this code solves the problem.