Firebase数据库中的子参考

Question

我只想检索活动id1和id2，以便将其保存在列表中。但是，我这样做，最后一行返回整个对象引用

{Category 1={name=joy, Sprint={Activities={id2=23r90u9329r,
id1=jf92jf943jf94j3f9}, startingDate =08092017, sprintOverallScore=1,
Ending Date=08092017, Number of Weeks=2}}

我如何才能获得Activities={id2=23r90u9329r, id1=jf92jf943jf94j3f9}

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_test);


        //getting the reference of artists node
        databaseCategories = FirebaseDatabase.getInstance().getReference("Categories");


databaseCategories.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                //clear the list
                currentCategories.clear();

                //iterate through all the node
                for(DataSnapshot postsnapshot: dataSnapshot.getChildren()){


                    //Category j = postsnapshot.getValue(Category.class);

                    System.out.println("test: " + postsnapshot.getValue());

我尝试创建这样的2个数据库引用，但它返回NULL：

DatabaseReference databaseCategories, databaseCategories2;

             ....


databaseCategories.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                //clear the list
                currentCategories.clear();

                //iterate through all the node
                for(DataSnapshot postsnapshot: dataSnapshot.getChildren()){

                    String g = postsnapshot.getKey(); //get the id

                    databaseCategories2 = FirebaseDatabase.getInstance().getReference("CategoryNew").child(g+"").child("Category 1").child("Sprint").child("Activities");


                    databaseCategories2.addValueEventListener(new ValueEventListener() {
                        @Override
                        public void onDataChange(DataSnapshot dataSnapshot) {

                            System.out.println("test new: " + dataSnapshot.getValue()); //returning NULL

                        }

                        @Override
                        public void onCancelled(DatabaseError databaseError) {

                        }
                    });

Answer 1

由于您在/Categories上附加了一个监听器，因此onDataChange将调用dataSnapshot，其中包含完整值。你循环遍历其子级，这会让你降低一级，但是postSnapshot仍然是hsdhs...下的整个数据。您还需要在代码中导航快照的其余部分，以进入活动：

databaseCategories = FirebaseDatabase.getInstance().getReference("Categories");

databaseCategories.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        //iterate through the categories
        for(DataSnapshot categorySnapshot: dataSnapshot.getChildren()){
            DataSnapshot activitiesSnapshot = categorySnapshot.child("Category1/Sprint/Activities");
            for(DataSnapshot activitySnapshot: activitiesSnapshot.getChildren()){
                System.out.println(activitySnapshot.getKey()+": "+activitySnapshot.getValue(String.class));
            }
        }
    }

    ...

请注意，这非常浪费，包括您必须编写的代码量和检索的数据量。如果您的应用中有一个常见的用例来查找某个类别的活动，请考虑将这些用作顶级列表存储：

CategoryActivities
  CategoryId
    ActivityId1: "value 1"
    ActivityId2: "value 2"

使用此结构，您可以更轻松地精确读取所需的数据，从而节省数据传输和深层嵌套代码。