How to refer sub child in realtime firebase in android?

Question

I am going to find student attendance by matching the registration number stored in realtime database. I am trying hard to get the student's registration no and the date when student visited school.

final FirebaseDatabase databs= FirebaseDatabase.getInstance();
DatabaseReference p = databs.getReference("StudentAttend");
Query q= p.child("reg_no").orderByChild("date").equalTo(Parent_HomePage.childR)

Here is my query to fetch them. But the issue is that registration number and date have no names in database, they are keys. Kindly help me to fetch these attributes from database. Secondly kindly share Data snapshot code because my view doesn't show anything.

Thanks

What do you know about the student? The ID or Date or something? — Dharmaraj, May 17 '21 at 05:19
Under "StudentAttend" I cannot see any "reg_no" and also no "date" property. Is it the right schema? — Alex Mamo, May 17 '21 at 10:30
@AlexMamo Yes this is the issue. 14-03-2021 is date and 165 is reg_no. There are not any name of these attributes. I don't came to know how I will approach them. — Rahat Batool, May 17 '21 at 10:41
@AlexMamo Basically In my application Parent is able to check child's attendance. Here child attendance is saved. Whenever parent will check than all the dates when 165 attended the school will be displayed to parent. — Rahat Batool, May 17 '21 at 10:51
I'm sorry but I didn't really understand. What is the exact data you want to get? — Alex Mamo, May 17 '21 at 11:02
@AlexMamo You simply help me to write query to get the dates (14-03-2021) and the reg_no (165) from firebase to my listview — Rahat Batool, May 17 '21 at 11:10
@AlexMamo Kindly focus on firebase picture. I want to show dates in my list view whose child is 165. — Rahat Batool, May 17 '21 at 11:33

SABANTO · Answer 1 · 2021-05-17T06:31:51.123

To get a key from realtime database you use getKey(). But i have to say you have to structure your data better. Also your query won't work as dates come before reg no. And ordering by date as shown will lead to issues for example 03-03-2021 will come before 20-02-2021.

       final FirebaseDatabase databs= FirebaseDatabase.getInstance();
       DatabaseReference p = databs.getReference("StudentAttend");

       p.addListenerForSingleValueEvent(new ValueEventListener() {
           @Override
           public void onDataChange(DataSnapshot dns) {
               for (DataSnapshot snapshot : dns.getChildren()) {
                   String key = dns.getKey(); //this will get the date
                   //to get the student number within a date use above key
                   p.child(key).addListenerForSingleValueEvent(new ValueEventListener() {
                         @Override
                         public void onDataChange(DataSnapshot dns) {
                             for (DataSnapshot snapshot : dns.getChildren()) {
                                 String key = dns.getKey(); //this will get the reg_no
                                 //get pick and drop here

                    }
              }}}
                        
         }

To query data when you have a date

           final FirebaseDatabase databs= FirebaseDatabase.getInstance();
           DatabaseReference p = databs.getReference("StudentAttend").child(date_you_have); //then add listener here

If you don't have a date but have a reg no, follow above code to get the date key, then run query in there with

p.child(key).child(regnoyouhvae);

But i suggest you go over documentation again and read some articles on structuring your data, queries and more.

Dates come before reg_no because every student attendance is saved relative to the dates. On which date, which students were present. — Rahat Batool, May 17 '21 at 10:43

score 1 · Answer 2 · answered May 17 '21 at 12:15

1

According to your last comment:

I want to show dates in my ListView whose child is 165.

To display the keys (dates) if the DataSnapshot object contains a child called "165", please use the following lines of code:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference studentAttendRef = rootRef.child("StudentAttend");
studentAttendRef.get().addOnCompleteListener(new OnCompleteListener<DataSnapshot>() {
    @Override
    public void onComplete(@NonNull Task<DataSnapshot> task) {
        if (task.isSuccessful()) {
            for (DataSnapshot ds : task.getResult().getChildren()) {
                if (ds.child("165").exists()) {
                    Log.d("TAG", ds.getKey());
                }
            }
        } else {
            Log.d("TAG", task.getException().getMessage()); //Don't ignore potential errors!
        }
    }
});

The result in the logcat will be:

14-03-2021
15-04-2021

To be able to display these dates in a ListView, please see my answer from the following post:

Showing Firebase data in ListView

answered May 17 '21 at 12:15

Alex Mamo

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Yes you got it. Now I want to display dates regardless of reg_no. It may be 165 or 206 according to user input. – Rahat Batool May 17 '21 at 12:31
Simply pass a variable that can hold the value that you need. Something like "ds.child(numberYouNeed)" and **not** a hardcoded value, right? – Alex Mamo May 17 '21 at 12:47
Ok, Thanks Alot – Rahat Batool May 17 '21 at 12:51
I typed the above code and .get() is not accepted. It shows error on .get() `studentAttendRef.get().addOnCompleteListener(new OnCompleteListener()` – Rahat Batool May 17 '21 at 13:02
Most likely you don't have the latest version of the Firebase Realtime Database. Update it and tell me if it works. – Alex Mamo May 17 '21 at 14:12

How to refer sub child in realtime firebase in android?

2 Answers2