如何在 Firebase 实时数据库（Android 开发）中获取特定子项的键值？

Question

How to get the key value of a specific child in the Firebase Realtime database (Android Development)? I want to get the value of a specific key according to the selected subject. Suppose if the user selects java then I want to get ' -MnLZAaa1pmVpRusEG8s ' (in the blue box). I tried to get the subject name and subject unique keys in the list and then compare it with the string selected by the user, but don't know why I am getting a null value. How can I do it?

public String getSubjectUuid() {
    List<String> subjectNamesList = new ArrayList<>();
    List<String> subjectUuidList = new ArrayList<>();

    DatabaseReference addedSubjectReference = FirebaseDatabase
            .getInstance()
            .getReference(classCreatorPhoneNo + "/" + "class_list" + "/" + classUuid + "/" + "added_subject");

    addedSubjectReference
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
                    if (snapshot.exists()) {
                        subjectNamesList.add(snapshot.child("subject_name").getValue(String.class));
                        subjectUuidList.add(snapshot.child("subject_uuid").getValue(String.class));
                    }
                }

                @Override
                public void onChildChanged(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
                }

                @Override
                public void onChildRemoved(@NonNull DataSnapshot snapshot) {
                }

                @Override
                public void onChildMoved(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
                }

                @Override
                public void onCancelled(@NonNull DatabaseError error) {
                }
            });

    for (int i = 0; i < subjectNamesList.size(); i++) {
        if (selectedSubject.equals(subjectNamesList.get(i))) {
            classKey = subjectUuidList.get(i);
            break;
        }
    }
    
    return classKey;
}

Answer 1

You're looking to use a query that orders/filters on your subject_name field. Something like this should do the trick:

addedSubjectReference
    .orderByChild("subject_name")      // 👈 First order on a property
    .equalTo("java")                   // 👈 Then filter on a value
    .addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
            //if (snapshot.exists()) { // 👈 This is not needed inside `onChildAdded`
                subjectNamesList.add(snapshot.child("subject_name").getValue(String.class));
                subjectUuidList.add(snapshot.child("subject_uuid").getValue(String.class));
            //}
        }