如何获取Firebase实时数据库中特定child的键值（Android开发）？

Question

如何在Firebase实时数据库中获取特定child的键值（Android开发）？我想根据所选主题获取特定键的值。假设如果用户选择 java 那么我想得到 '-MnLZAaa1pmVpRusEG8s'（在蓝色框中）。我试图获取列表中的主题名称和主题唯一键，然后将其与用户选择的字符串进行比较，但不知道为什么我得到的是空值。我该怎么做？

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

您正在寻找 use a query orders/filters 在您的 subject_name 字段中。这样的事情应该可以解决问题：

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));
            //}
        }

如何获取Firebase实时数据库中特定child的键值（Android开发）？

How to get the key value of a specific child in the Firebase Realtime database (Android Development)?

java

android

firebase

firebase-realtime-database