Firebase中的嵌套列表 [英] Nested Lists in Firebase

问题描述

试图理解如何在Firebase中实现嵌套列表。

问题可以简化为：1对N消息传递系统，对于每个消息，维护已收到和阅读该消息的用户列表。

阅读Firebase中阵列的最佳实践。试图避免数组，因为我有同时写入，他们只是在这里似乎不是一个好的选择。

目前试图通过在每个消息下存储子树，每个子树都是在消息 X 上接收，阅读或以其他方式执行某个操作 X

 msgid0：{
authorID：uid0，
msg：消息文本，
ReceivedBy：{
 uid1：true，
 uid2：true 
} 
ReadBy：{
 uid1：true 
 
 
 
 
 
 
 $ b问题：是否可以像这样放置一个嵌套的数据结构，直接到一个单一的对象？
 
 
 
我粗暴地尝试一下测试用例：
 $ b $ 
  public class Message {
 private Long authorID; 
 private String msg; 
私人清单< String> receivedBy; 
私人清单< String> readBy; （DataSnapshot i：dataSnapshot）
 
 
 ref.addValueEventListener（new ValueEventListener（）{
 @Override 
 public void onDataChange（DataSnapshot dataSnapshot）{
 .getChildren（））{
 Message_FB msg = i.getValue（Message_FB.class）; 
} 
} 
} 
  pre> 
 
 
但是失败：
 $ b 
 
 
引起：com.fasterxml。 jackson.databind.JsonMappingException：
可以不反序列化java.util.ArrayList的实例，但不包括START_OBJECT 
令牌
 $ / 
 $ div class =h2_lin >解决方案

啊，只需要使用 Map 而不是 List p>



在我的例子中：



  public class Message {
 private Long authorID; 
 private String msg; 
 private Map< String，Boolean> receivedBy; 
 private Map< String，Boolean> readBy; 
} 
  

现在我明白了如何使用recursiv e结构与Firebase。猜猜大多数事情，这很容易 - 一旦你知道如何。

Trying to understand how to implement nested lists in Firebase.

Problem reducible to: a 1-to-N messaging system, where, for each message, you wish to maintain a list of users who have received, and read, that message.

Have read "Best Practices for Arrays in Firebase". Trying to avoid arrays, as I have simultaneous writes, and they just don't seem a good choice here.

Currently trying to achieve this by storing subtree's under each message, each subtree being a list of users who have received, read, or otherwise performed some action X on message Y

"msgid0" : {
        "authorID": "uid0",
        "msg"     : "message text",
        "ReceivedBy": {
           uid1 : true,
           uid2 : true
         }
        "ReadBy" :    {
           uid1 : true
        }
}

Question: Is it possible to put a nested datastructure like this, directly into a single object?

I am crudely trying a test case with the following:

public class Message {
  private Long authorID;
  private String msg;
  private List<String> receivedBy;
  private List<String> readBy;
}

ref.addValueEventListener(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot dataSnapshot) {
    for(DataSnapshot i: dataSnapshot.getChildren()){
      Message_FB msg = i.getValue(Message_FB.class);
    }
  }
}

But it fails with:

Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token

解决方案

Ah, just needed to use Map instead of List.

In my case:

public class Message {
  private Long authorID;
  private String msg;
  private Map<String, Boolean> receivedBy;
  private Map<String, Boolean> readBy;
}

Now I understand how to use recursive structures with Firebase. Guess like most things, it is easy -- once you know how.

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