flutter/firebase数据库中的简单查询 [英] A simple Query in flutter/firebase database

问题描述

我尝试体验Flubase的Firebase Live数据库. 我只是想在firebase响应的datasnapshot中获取一个值.

I try to experience Firebase Live database with flutter. I just would like to get a value in the datasnapshot of the firebase response.

我的Firebase

My Firebase

我的代码

static Future<User> getUser(String userKey) async {
Completer<User> completer = new Completer<User>();

String accountKey = await Preferences.getAccountKey();

FirebaseDatabase.instance
    .reference()
    .child("accounts")
    .child(accountKey)
    .child("users")
    .childOrderBy("Group_id")
    .equals("54")
    .once()
    .then((DataSnapshot snapshot) {
  var user = new User.fromSnapShot(snapshot.key, snapshot.value);
  completer.complete(user);
});

return completer.future;
  }
}

class User {
  final String key;
  String firstName;

  Todo.fromJson(this.key, Map data) {
    firstname= data['Firstname'];
    if (firstname== null) {
      firstname= '';
    }
  }
}

我的名字为Null. 我想我应该导航到snapshot.value的子级.但是无法通过foreach或Map()...来管理.

I got Null value for firstname. I guess I should navigate to the child of snapshot.value. But impossible to manage with foreach, or Map(), ...

亲切的问候，杰罗姆

推荐答案

您正在使用查询和查询文档(

You are querying with a query and the documentation for Queries (here in JavaScript, but it is valid for all languages), says that "even when there is only a single match for the query, the snapshot is still a list; it just contains a single item. To access the item, you need to loop over the result."

我不知道您应该如何在Flutter/Dart中循环遍历快照的子级，但是您应该执行以下操作(在JavaScript中):

I don't know exactly how you should loop, in Flutter/Dart, over the children of the snapshot but you should do something like the following (in JavaScript):

  snapshot.forEach(function(childSnapshot) {
    var childKey = childSnapshot.key;
    var childData = childSnapshot.val();
    // ...
  });

，并假设您的查询仅返回一条记录(一条匹配项")，则在执行操作时使用子快照

and assuming that your query returns only one record ("one single match"), use the child snapshot when you do

var user = new User.fromSnapShot(childSnapshot.key, childSnapshot.value);

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