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

Posted 2023-05-07

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

【英文标题】：How to get the key value of a specific child in the Firebase Realtime database (Android Development)?

【发布时间】：2021-12-15 16:16:33

【问题描述】：

如何获取 Firebase 实时数据库中特定子项的键值（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 那么我想获得 '-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一个空值。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;

【参考方案1】：

您正在寻找use a query 对您的subject_name 字段进行排序/过滤。像这样的东西应该可以解决问题：

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));
            //