Firebase 如何将来自Flatter中两个Firestore集合的数据连接起来？

我在Flatter中使用Firestore有一个聊天应用程序，我有两个主要收藏：

• 聊天记录

  ，可键入自动ID，并具有

  消息

  、

  时间戳

  和

  uid

  字段

• 用户

  ，输入

  uid

  ，并有一个

  名称

  字段

在我的应用程序中，我使用以下小部件显示一个消息列表（来自

消息
集合）：

class ChatList extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    var messagesSnapshot = Firestore.instance.collection("chat").orderBy("timestamp", descending: true).snapshots();
    var streamBuilder = StreamBuilder<QuerySnapshot>(
          stream: messagesSnapshot,
          builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> querySnapshot) {
            if (querySnapshot.hasError)
              return new Text('Error: ${querySnapshot.error}');
            switch (querySnapshot.connectionState) {
              case ConnectionState.waiting: return new Text("Loading...");
              default:
                return new ListView(
                  children: querySnapshot.data.documents.map((DocumentSnapshot doc) {
                    return new ListTile(
                      title: new Text(doc['message']),
                      subtitle: new Text(DateTime.fromMillisecondsSinceEpoch(doc['timestamp']).toString()),
                    );
                  }).toList()
                );
            }
          }
        );
        return streamBuilder;
  }
}

类聊天列表扩展了无状态小部件{
@凌驾
小部件构建（构建上下文）{
var messagesSnapshot=Firestore.instance.collection（“chat”）.orderBy（“timestamp”，降序：true）.snapshots（）；
var streamBuilder=streamBuilder(
流：messagesSnapshot，
生成器：（BuildContext上下文，AsyncSnapshot querySnapshot）{
if（querySnapshot.hasError）
返回新文本（'Error:${querySnapshot.Error}'）；
开关（querySnapshot.connectionState）{
case ConnectionState.waiting：返回新文本（“加载…”）；
违约：
返回新的ListView(
子项：querySnapshot.data.documents.map（（DocumentSnapshot文档）{
返回新的ListTile(
标题：新文本（doc['message']），
字幕：新文本（DateTime.FromMillisSecondsSinceEpoch（doc['timestamp']）.toString（）），
);
})托利斯先生（）
);
}
}
);
返回streamBuilder；
}
}

但是现在我想显示每条消息的用户名（来自
users
集合）

我通常称之为客户端连接，尽管我不确定flifter是否有特定的名称

我找到了一种方法（我在下面发布了），但不知道是否有另一种/更好/更惯用的方法在颤振中执行这种操作

那么：在flatter中，查找上述结构中每条消息的用户名的惯用方法是什么呢？
我得到的第一个解决方案是嵌套两个
StreamBuilder
实例，每个集合/查询一个

class ChatList extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    var messagesSnapshot = Firestore.instance.collection("chat").orderBy("timestamp", descending: true).snapshots();
    var usersSnapshot = Firestore.instance.collection("users").snapshots();
    var streamBuilder = StreamBuilder<QuerySnapshot>(
      stream: messagesSnapshot,
      builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> messagesSnapshot) {
        return StreamBuilder(
          stream: usersSnapshot,
          builder: (context, usersSnapshot) {
            if (messagesSnapshot.hasError || usersSnapshot.hasError || !usersSnapshot.hasData)
              return new Text('Error: ${messagesSnapshot.error}, ${usersSnapshot.error}');
            switch (messagesSnapshot.connectionState) {
              case ConnectionState.waiting: return new Text("Loading...");
              default:
                return new ListView(
                  children: messagesSnapshot.data.documents.map((DocumentSnapshot doc) {
                    var user = "";
                    if (doc['uid'] != null && usersSnapshot.data != null) {
                      user = doc['uid'];
                      print('Looking for user $user');
                      user = usersSnapshot.data.documents.firstWhere((userDoc) => userDoc.documentID == user).data["name"];
                    }
                    return new ListTile(
                      title: new Text(doc['message']),
                      subtitle: new Text(DateTime.fromMillisecondsSinceEpoch(doc['timestamp']).toString()
                                          +"\n"+user),
                    );
                  }).toList()
                );
            }
        });
      }
    );
    return streamBuilder;
  }
}

类聊天列表扩展了无状态小部件{
@凌驾
小部件构建（构建上下文）{
var messagesSnapshot=Firestore.instance.collection（“chat”）.orderBy（“timestamp”，降序：true）.snapshots（）；
var usersSnapshot=Firestore.instance.collection（“users”）.snapshot（）；
var streamBuilder=streamBuilder(
流：messagesSnapshot，
生成器：（BuildContext上下文，异步快照消息snapshot）{
返回流生成器(
流：usersSnapshot，
生成器：（上下文，usersSnapshot）{
if（messagesSnapshot.hasError | | | usersSnapshot.hasError | | |！usersSnapshot.hasData）
返回新文本（'Error:${messagesSnapshot.Error}，${usersSnapshot.Error}'）；
开关（消息snapshot.connectionState）{
case ConnectionState.waiting：返回新文本（“加载…”）；
违约：
返回新的ListView(
子项：messagesSnapshot.data.documents.map（（DocumentSnapshot文档）{
var user=“”；
if（doc['uid']！=null&&usersSnapshot.data！=null）{
user=doc['uid']；
打印（“查找用户$user”）；
user=usersSnapshot.data.documents.firstWhere（（userDoc）=>userDoc.documentID==user.data[“name”]；
}
返回新的ListTile(
标题：新文本（doc['message']），
字幕：新文本（DateTime.FromMillisSecondsSinceEpoch（doc['timestamp']）。toString（）
+“\n”+用户），
);
})托利斯先生（）
);
}
});
}
);
返回streamBuilder；
}
}

正如我在问题中所说的，我知道这个解决方案不是很好，但至少它是有效的

我发现这方面存在一些问题：


它加载所有用户，而不仅仅是发布消息的用户。在小数据集中，这不会是一个问题，但随着我获得更多的消息/用户（并使用查询显示其中的一个子集），我将加载越来越多没有发布任何消息的用户
对于两个构建器的嵌套，代码的可读性不是很强。我怀疑这是否是惯用的颤振

如果你知道一个更好的解决方案，请作为答案发布。
我得到了另一个版本，看起来比这个版本稍微好一点

在这里，我使用一个专用的
Message
类来保存Message
文档
和可选的关联用户
文档
中的信息，并在自定义方法中隔离数据加载

class Message {
  final message;
  final timestamp;
  final uid;
  final user;
  const Message(this.message, this.timestamp, this.uid, this.user);
}
class ChatList extends StatelessWidget {
  Stream<List<Message>> getData() async* {
    var messagesStream = Firestore.instance.collection("chat").orderBy("timestamp", descending: true).snapshots();
    var messages = List<Message>();
    await for (var messagesSnapshot in messagesStream) {
      for (var messageDoc in messagesSnapshot.documents) {
        var message;
        if (messageDoc["uid"] != null) {
          var userSnapshot = await Firestore.instance.collection("users").document(messageDoc["uid"]).get();
          message = Message(messageDoc["message"], messageDoc["timestamp"], messageDoc["uid"], userSnapshot["name"]);
        }
        else {
          message = Message(messageDoc["message"], messageDoc["timestamp"], "", "");
        }
        messages.add(message);
      }
      yield messages;
    }
  }
  @override
  Widget build(BuildContext context) {
    var streamBuilder = StreamBuilder<List<Message>>(
          stream: getData(),
          builder: (BuildContext context, AsyncSnapshot<List<Message>> messagesSnapshot) {
            if (messagesSnapshot.hasError)
              return new Text('Error: ${messagesSnapshot.error}');
            switch (messagesSnapshot.connectionState) {
              case ConnectionState.waiting: return new Text("Loading...");
              default:
                return new ListView(
                  children: messagesSnapshot.data.map((Message msg) {
                    return new ListTile(
                      title: new Text(msg.message),
                      subtitle: new Text(DateTime.fromMillisecondsSinceEpoch(msg.timestamp).toString()
                                         +"\n"+(msg.user ?? msg.uid)),
                    );
                  }).toList()
                );
            }
          }
        );
        return streamBuilder;
  }
}

类消息{
最后信息；
最终时间戳；
最终uid；
最终用户；
const Message（this.Message、this.timestamp、this.uid、this.user）；
}
类ChatList扩展了无状态小部件{
流getData（）异步*{
var messagesStream=Firestore.instance.collection（“chat”）.orderBy（“timestamp”，降序：true）.snapshots（）；
var messages=List（）；
等待（消息流中的var messagesSnapshot）{
for（messagesSnapshot.documents中的var messageDoc）{
var消息；
if（messageDoc[“uid”！=null）{
var userSnapshot=await Firestore.instance.collection（“users”）.document（messageDoc[“uid”]）.get（）；
message=message（messageDoc[“message”]、messageDoc[“timestamp”]、messageDoc[“uid”]、userSnapshot[“name”]）；
}
否则{
message=message（messageDoc[“message”]、messageDoc[“timestamp”]、“”、“”）；
}
消息。添加（消息）；
}
产生信息；
}
}
@凌驾
小部件构建（构建上下文）{
var streamBuilder=streamBuilder(
流：getData（），
生成器：（BuildContext上下文，AsyncSnapsho）
import 'dart:async';
import 'dart:math' show Random;

final random = Random();

const messageList = [
  {
    'message': 'Message 1',
    'timestamp': 1,
    'uid': 1,
  },
  {
    'message': 'Message 2',
    'timestamp': 2,
    'uid': 2,
  },
  {
    'message': 'Message 3',
    'timestamp': 3,
    'uid': 2,
  },
];

const userList = {
  1: 'User 1',
  2: 'User 2',
  3: 'User 3',
};

class Message {
  final String message;
  final int timestamp;
  final int uid;
  final String user;
  const Message(this.message, this.timestamp, this.uid, this.user);

  @override
  String toString() => '$user => $message';
}

// Mimic a stream of a list of messages
Stream<List<Map<String, dynamic>>> getServerMessagesMock() async* {
  yield messageList;
  while (true) {
    await Future.delayed(Duration(seconds: random.nextInt(3) + 1));
    yield messageList;
  }
}

// Mimic asynchronously fetching a user
Future<String> userMock(int uid) => userList.containsKey(uid)
    ? Future.delayed(
        Duration(milliseconds: 100 + random.nextInt(100)),
        () => userList[uid],
      )
    : Future.value(null);

// Transform the contents of a stream asynchronously
Stream<List<Message>> getMessagesStream() => getServerMessagesMock()
    .asyncMap<List<Message>>((messageList) => Future.wait(
          messageList.map<Future<Message>>(
            (m) async => Message(
              m['message'],
              m['timestamp'],
              m['uid'],
              await userMock(m['uid']),
            ),
          ),
        ));

void main() async {
  print('Streams with async transforms test');
  await for (var messages in getMessagesStream()) {
    messages.forEach(print);
  }
}

import 'package:rxdart/rxdart.dart';

class Messages {
  final String messages;
  final DateTime timestamp;
  final String uid;
  final DocumentReference reference;

  Messages.fromMap(Map<String, dynamic> map, {this.reference})
      : messages = map['messages'],
        timestamp = (map['timestamp'] as Timestamp)?.toDate(),
        uid = map['uid'];

  Messages.fromSnapshot(DocumentSnapshot snapshot)
      : this.fromMap(snapshot.data, reference: snapshot.reference);

  @override
  String toString() {
    return 'Messages{messages: $messages, timestamp: $timestamp, uid: $uid, reference: $reference}';
  }
}

class Users {
  final String name;
  final DocumentReference reference;

  Users.fromMap(Map<String, dynamic> map, {this.reference})
      : name = map['name'];

  Users.fromSnapshot(DocumentSnapshot snapshot)
      : this.fromMap(snapshot.data, reference: snapshot.reference);

  @override
  String toString() {
    return 'Users{name: $name, reference: $reference}';
  }
}

class CombineStream {
  final Messages messages;
  final Users users;

  CombineStream(this.messages, this.users);
}

Stream<List<CombineStream>> _combineStream;

@override
  void initState() {
    super.initState();
    _combineStream = Observable(Firestore.instance
        .collection('chat')
        .orderBy("timestamp", descending: true)
        .snapshots())
        .map((convert) {
      return convert.documents.map((f) {

        Stream<Messages> messages = Observable.just(f)
            .map<Messages>((document) => Messages.fromSnapshot(document));

        Stream<Users> user = Firestore.instance
            .collection("users")
            .document(f.data['uid'])
            .snapshots()
            .map<Users>((document) => Users.fromSnapshot(document));

        return Observable.combineLatest2(
            messages, user, (messages, user) => CombineStream(messages, user));
      });
    }).switchMap((observables) {
      return observables.length > 0
          ? Observable.combineLatestList(observables)
          : Observable.just([]);
    })
}

@override
      void initState() {
        super.initState();
        _combineStream = Firestore.instance
            .collection('chat')
            .orderBy("timestamp", descending: true)
            .snapshots()
            .map((convert) {
          return convert.documents.map((f) {

            Stream<Messages> messages = Stream.value(f)
                .map<Messages>((document) => Messages.fromSnapshot(document));

            Stream<Users> user = Firestore.instance
                .collection("users")
                .document(f.data['uid'])
                .snapshots()
                .map<Users>((document) => Users.fromSnapshot(document));

            return Rx.combineLatest2(
                messages, user, (messages, user) => CombineStream(messages, user));
          });
        }).switchMap((observables) {
          return observables.length > 0
              ? Rx.combineLatestList(observables)
              : Stream.value([]);
        })
    }

class ChatBloc {
  final Firestore firestore = Firestore.instance;
  final Map<String, String> userMap = HashMap<String, String>();

  Stream<List<Message>> get messages async* {
    final messagesStream = Firestore.instance.collection('chat').orderBy('timestamp', descending: true).snapshots();
    var messages = List<Message>();
    await for (var messagesSnapshot in messagesStream) {
      for (var messageDoc in messagesSnapshot.documents) {
        final userUid = messageDoc['uid'];
        var message;

        if (userUid != null) {
          // get user data if not in map
          if (userMap.containsKey(userUid)) {
            message = Message(messageDoc['message'], messageDoc['timestamp'], userUid, userMap[userUid]);
          } else {
            final userSnapshot = await Firestore.instance.collection('users').document(userUid).get();
            message = Message(messageDoc['message'], messageDoc['timestamp'], userUid, userSnapshot['name']);
            // add entry to map
            userMap[userUid] = userSnapshot['name'];
          }
        } else {
          message =
              Message(messageDoc['message'], messageDoc['timestamp'], '', '');
        }
        messages.add(message);
      }
      yield messages;
    }
  }
}

class ChatList extends StatelessWidget {
  final ChatBloc chatBloc = ChatBloc();

  @override
  Widget build(BuildContext context) {
    return StreamBuilder<List<Message>>(
        stream: chatBloc.messages,
        builder: (BuildContext context, AsyncSnapshot<List<Message>> messagesSnapshot) {
          if (messagesSnapshot.hasError)
            return new Text('Error: ${messagesSnapshot.error}');
          switch (messagesSnapshot.connectionState) {
            case ConnectionState.waiting:
              return new Text('Loading...');
            default:
              return new ListView(children: messagesSnapshot.data.map((Message msg) {
                return new ListTile(
                  title: new Text(msg.message),
                  subtitle: new Text('${msg.timestamp}\n${(msg.user ?? msg.uid)}'),
                );
              }).toList());
          }
        });
  }
}

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/widgets.dart';
import 'package:rxdart/rxdart.dart';

class MessageWidget extends StatelessWidget {
  // final chatStream = Firestore.instance.collection('chat').snapshots();
  // final userStream = Firestore.instance.collection('users').snapshots();
  Stream<QuerySnapshot> chatStream;
  Stream<QuerySnapshot> userStream;

  MessageWidget(this.chatStream, this.userStream);

  @override
  Widget build(BuildContext context) {
    Observable<List<QuerySnapshot>> combinedStream = Observable.combineLatest2(
        chatStream, userStream, (messages, users) => [messages, users]);

    return StreamBuilder(
        stream: combinedStream,
        builder: (_, AsyncSnapshot<List<QuerySnapshot>> snapshots) {
          if (snapshots.hasData) {
            List<DocumentSnapshot> chats = snapshots.data[0].documents;

            // It would be more efficient to convert this list of user documents
            // to a map keyed on the uid which will allow quicker user lookup.
            List<DocumentSnapshot> users = snapshots.data[1].documents;

            return ListView.builder(itemBuilder: (_, index) {
              return Center(
                child: Column(
                  children: <Widget>[
                    Text(chats[index]['message']),
                    Text(getUserName(users, chats[index]['uid'])),
                  ],
                ),
              );
            });
          } else {
            return Text('loading...');
          }
        });
  }

  // This does a linear search through the list of users. However a map
  // could be used to make the finding of the user's name more efficient.
  String getUserName(List<DocumentSnapshot> users, String uid) {
    for (final user in users) {
      if (user['uid'] == uid) {
        return user['name'];
      }
    }
    return 'unknown';
  }
}