Retrieve count data from Firebase Java

I am trying to count the number of children in my DB table that meet a certain condition. If the isSeen column equals false then I want to count that, if it equals true I don’t want to count it.

Currently, it’s not working but if I change the query from Query query = usersRef.orderByChild("isSeen").equalTo(true); to Query query = usersRef.orderByChild("isSeen"); I get a number but it’s not the correct way. Can someone please help me?

Rules:

{   
"rules": {     
  ".read": true,     
    ".write": true,     
      "Messages": {        
          ".indexOn": "isSeen"       
       }     
  }   
} 

"Messages": {
"b3vYlKZFrje0e3wHyBlWIK4ooK93": {
  "DIt5bGqw2WS4eGHNqQJKxZSn3B72": {
    "-N8NCgnwX6V7ghfGlcWS": {
      "dateAdded": 1659337356887,
      "date_time": "Aug-01-2022 3:02:36 AM",
      "from": "DIt5bGqw2WS4eGHNqQJKxZSn3B72",
      "isSeen": true,
      "message": "Yoo",
      "to": "b3vYlKZFrje0e3wHyBlWIK4ooK93",
      "type": "text"
    },
    "-N99iQjlMfeyOM_VCAEB": {
      "dateAdded": 1660184797462,
      "date_time": "Aug-10-2022 10:26:37 PM",
      "from": "DIt5bGqw2WS4eGHNqQJKxZSn3B72",
      "isSeen": true,
      "message": "Wassup",
      "to": "b3vYlKZFrje0e3wHyBlWIK4ooK93",
      "type": "text"
    }
  }
}
},

---

DatabaseReference usersRef = FirebaseDatabase.getInstance().getReference("Messages");
    Query query = usersRef.child(firebaseUser.getUid()).orderByChild("isSeen").equalTo(true);
    query.get().addOnCompleteListener(new OnCompleteListener<DataSnapshot>() {
        @Override
        public void onComplete(@NonNull Task<DataSnapshot> task) {
            if (task.isSuccessful()) {
                long count = task.getResult().getChildrenCount();
                Log.d("TAG1", "count: " + count);
            } else {
                Log.d("TAG2", task.getException().getMessage()); //Never ignore potential errors!
            }
        }
    });

Error message:

D/TAG2: Index not defined, add ".indexOn": "isSeen", for path "/Messages/b3vYlKZFrje0e3wHyBlWIK4ooK93", to the rules

Database schema

Answer

When you’re using the following query:

Query query = usersRef.orderByChild("isSeen").equalTo(true);

Firebase will always return the exact data you are querying, meaning that you’ll get all elements that have the isSeen field set to true. Please note that there is no way you can query by a negation. So something like this is not possible:

Query query = usersRef.orderByChild("isSeen").notEqualTo(true);
//                                               👆

According to your comment in which you say that you don’t have any elements where the isSeen field is set to true, then your query will yield no results, and that’s the expected behavior.

While @TimothyPham’s answer will work, using getChildrenCount() might be the best solution. Why? Because if you have a lot of messages this operation requires you to read all of them in order to provide a number. The best solution I can think of would be to increment/decrement a counter as explained in my answer from the following post:

• How to save users score in firebase and retrieve it in real-time in Android studio

Edit:

Query query = usersRef.orderByChild("isSeen").equalTo(true);
query.get().addOnCompleteListener(new OnCompleteListener<DataSnapshot>() {
    @Override
    public void onComplete(@NonNull Task<DataSnapshot> task) {
        if (task.isSuccessful()) {
            long count = task.getResult().getChildrenCount();
            Log.d("TAG", "count: " + count);
        } else {
            Log.d("TAG", task.getException().getMessage()); //Never ignore potential errors!
        }
    }
});

But this code will only work if you have elements in the database that have he isSeen field is set to true.