java - 如何对多个 child 执行查询？

Question

我正在尝试进行一项 Activity ，以显示类并将它们列在回收站 View 中。但是，我在从 firebase 查询数据时遇到问题。在我展示我的数据库结构后，我会解释更多。这是我的数据库结构:

"Classes": {
    "-Li9nBUgnFmCWqUmwV5W": {
        "class_info": {
            "date_clasname": "August 6, 2019",
            "room_number": "131",
            "subject": "Science",
            "teacher": "Ms.Henry",
            "id": "-Li9nBUgnFmCWqUmwV5W"
        }
    },
    "-Li9n_IYwbTNPdNAsTAu": {
        "class_info": {
            "date_clasname": "August 6, 2019",
            "room_number": "131",
            "subject": "Math",
            "teacher": "Ms.Henry",
            "id": "-Li9n_IYwbTNPdNAsTAu"
        }
    },
    "-Li9naUipsa865NBpZjW": {
        "class_info": {
            "date_clasname": "August 6, 2019",
            "room_number": "131",
            "subject": "Other",
            "teacher": "Ms.Henry",
            "id": "-Li9naUipsa865NBpZjW"
        }
    },
    "-Li9nbpwp2qchwmZRDP9": {
        "class_info": {
            "date_clasname": "August 6, 2019",
            "room_number": "131",
            "subject": "Technology",
            "teacher": "Ms.Henry",
            "id": "-Li9nbpwp2qchwmZRDP9"
        }
    },
    "-Li9ne0NDyjcB4SIWi1z": {
        "class_info": {
            "date_clasname": "August 6, 2019",
            "room_number": "131",
            "subject": "Social Studies",
            "teacher": "Ms.Henry",
            "id": "-Li9ne0NDyjcB4SIWi1z"
        }
    }
},

我正在尝试按主题查询类(class)，并且只显示主题为“科学”的类(class)。我尝试了各种各样的事情，但仍然不知道该怎么做。这是我的 Activity 代码:

public class Science_classes extends AppCompatActivity {
    private FirebaseDatabase database;
    private  DatabaseReference myRef, newmf;
   private List<Listdata> list;
   private RecyclerView recyclerview;
  String class_value = null;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_science_classes);
    recyclerview = (RecyclerView) findViewById(R.id.rview);
    database = FirebaseDatabase.getInstance();
    myRef = database.getReference("Classes");
    myRef.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {


            String id = dataSnapshot.getKey();

            Query query = myRef.child(id).child("class_info").orderByChild("subject").equalTo("Science");
            query.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    list = new ArrayList<>();
                    for (DataSnapshot dataSnapshot1 : dataSnapshot.getChildren()) {

                        Class_model new_class = dataSnapshot1.getValue(Class_model.class);
                        String nameofclass = new_class.getDate_clasname();
                        String teacherofclass = new_class.getTeacher();
                        String roomnumberofclass = new_class.getRoom_number();
                        String class_key = new_class.getUid();
                        Listdata listdata = new Listdata(nameofclass, teacherofclass, roomnumberofclass, class_key);
                        //String name = userdetails.getName();
                        //String email = userdetails.getEmail();
                        //String address = userdetails.getAddress();
                        listdata.setDate_class(nameofclass);
                        listdata.setTeacher(teacherofclass);
                        listdata.setRnumber(roomnumberofclass);
                        list.add(listdata);
                    }
                    RecyclerviewAdapter2 recycler = new RecyclerviewAdapter2(list);
                    RecyclerView.LayoutManager layoutmanager = new LinearLayoutManager(Science_classes.this);
                    recyclerview.setLayoutManager(layoutmanager);
                    recyclerview.setItemAnimator(new DefaultItemAnimator());
                    recyclerview.setAdapter(recycler);
                }
                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {

                }
            });
        }



        @Override
        public void onChildChanged(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

        }

        @Override
        public void onChildRemoved(@NonNull DataSnapshot dataSnapshot) {

        }

        @Override
        public void onChildMoved(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

        }
    @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }
    });
}


    }

我想我的问题可能是我无法获取随机生成的 ID 并在我的 onChildAdded 中将其设置为我的 child 之前我的另一个 child “class_info”。谢谢!

Answer 1

您正在构建这样的查询:

Query query = myRef.child(id).child("class_info").orderByChild("subject").equalTo("Science");

这是说通过 id 找到一个 child (我认为它没有显示在你的 JSON 中)然后找到一个 child class_info，然后在那个下面找到 subject 每个 child 的属性并对其进行过滤。所以路径是$id/class_info/*/subject，其中*是你要过滤返回的子节点。

但是你实际拥有的是/Classes/*/class_info/subject。因此，您有一个节点 Classes，您希望返回其 class_info/subject 具有给定值的子节点。在代码中是这样的:

DatabaseReference classesRef = FirebaseDatabase.getInstance().getReference("Classes");

Query query = classesRef.orderByChild("class_info/subject").equalTo("Science");

query.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        list = new ArrayList<>();
        for (DataSnapshot dataSnapshot1 : dataSnapshot.getChildren()) {
            Class_model new_class = dataSnapshot1.getValue(Class_model.class);
            ...