如何获得Firebase节点的随机子级? [英] How does one get a random children of a Firebase node?

问题描述

首先，这是我的数据库结构:

我的目标是从"DE"节点获取一个随机的Question对象，以便以后显示它，并且由于没有内置的查询随机子对象的支持，我必须以某种方式自己从该迭代器中获取一个随机Object. /p>

目前，我有这段代码，但对如何将其串在一起感到困惑:

    DatabaseReference questionsRef = FirebaseDatabase.getInstance().getReference().child("questions").child("DE");

questionsRef.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        int questionCount = (int) dataSnapshot.getChildrenCount();
        int rand = random.nextInt(questionCount);
        Iterator itr = dataSnapshot.getChildren().iterator();
    }
    // onCancelled(){}
});

解决方案

基本上，您只需要做足够的itr.next()，直到迭代器位于第n个位置(其中n是nextInt())，然后您可以简单地使用getValue()获取所需的对象，下面的示例应该可以很好地展示它:

questionsRef = FirebaseDatabase.getInstance().getReference().child("questions").child("DE");

questionsRef.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        int questionCount = (int) dataSnapshot.getChildrenCount();
        int rand = random.nextInt(questionCount);
        Iterator itr = dataSnapshot.getChildren().iterator();

        for(int i = 0; i < rand; i++) {
            itr.next();
        }
        childSnapshot = (DataSnapshot) itr.next();
        question = childSnapshot.getValue(Question.class);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});

First off, here's my Database structure:

My goal was to get a random Question object from the "DE" node to later display it, and as there's no builtin support for querying a random child I have to get a random Object myself, from that iterator, somehow.

Currently, I have this code, but am confused on how to string it together:

    DatabaseReference questionsRef = FirebaseDatabase.getInstance().getReference().child("questions").child("DE");

questionsRef.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        int questionCount = (int) dataSnapshot.getChildrenCount();
        int rand = random.nextInt(questionCount);
        Iterator itr = dataSnapshot.getChildren().iterator();
    }
    // onCancelled(){}
});

解决方案

Basically, you just have to do enough itr.next() until the iterator is at the nth position (where n is the random number from nextInt()) and then you can simply get the Object you want with getValue(), the example below should show it well:

questionsRef = FirebaseDatabase.getInstance().getReference().child("questions").child("DE");

questionsRef.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        int questionCount = (int) dataSnapshot.getChildrenCount();
        int rand = random.nextInt(questionCount);
        Iterator itr = dataSnapshot.getChildren().iterator();

        for(int i = 0; i < rand; i++) {
            itr.next();
        }
        childSnapshot = (DataSnapshot) itr.next();
        question = childSnapshot.getValue(Question.class);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});

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