如何在 Firestore 中进行内部连接

Question

我想构建一个视图来在我的应用程序的列表视图中显示一些事件，如下所示：

我有这两张表：

用户

 

活动

但我不知道如何在表 USERS 和 EVENTS 之间进行“内部连接”...

我试过了：

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:project/Methods.dart';
import 'package:project/Views/CadastroUsuario.dart';
import 'dart:math';

class EventClass{
  String owner;
  String description;
  String city;
  String state;
  String place;
}

class EventsListing extends StatefulWidget {
  @override
  EventsListingState createState() => new EventsListingState();
}

class EventsListingState extends State<EventsListing> {
  List<EventClass> events;

  @override
  void initState() {
    super.initState();
    events = new List<EventClass>();
  }

  void buildEventClass(DocumentSnapshot doc) async {
    EventClass oneEvent = new EventClass();

    DocumentReference document = Firestore.instance.collection("users").document(doc["userid"]);

    document.get().then((DocumentSnapshot snapshot){
      oneEvent.owner = snapshot["name"].toString();
    });
    oneEvent.description = doc["description"];
    oneEvent.place       = doc["place"];
    oneEvent.city        = doc["city"];
    oneEvent.state       = doc["state"];
    events.add(oneEvent);
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Events'), 
      ),
      body: new StreamBuilder(
        stream: Firestore.instance.collection("events").snapshots(),
        builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot){
          if (snapshot.connectionState == ConnectionState.waiting)
            return Text("Loading...");

          return new ListView(
            padding: EdgeInsets.only(left: 5.0, right: 5.0, top: 5.0),
            children: snapshot.data.documents.map((document){
               buildEventClass(document);
               return events.length == 0 ? new Card() : item(events.last);
              }).toList()
          );
        },
      ),
      floatingActionButton: new FloatingActionButton(
        tooltip: 'New',
        child: new Icon(Icons.add),
        onPressed: () async {
          Navigation navigation = new Navigation();
          navigation.navigaTo(context, CadastroUsuario());
         },
      ),
    );
  }

  Widget item(EventClass oneEvent) {
    return new Card(
      elevation: 4.0,
      child: new Column(
        children: <Widget>[
          new Row(
            children: <Widget>[
              new Column(
                children: <Widget>[
                  new Text(oneEvent.owner.toString(),
                    style: TextStyle(fontSize: 20.0),
                    overflow: TextOverflow.ellipsis,),
                ],
              ),
              new Column(
                children: <Widget>[

                ],
              )
            ],
          ),
          new Container(
            color: Colors.blue,
            height: 150.0,
          ),
          new Row(
            children: <Widget>[
              new Row( 
                children: <Widget>[
                  new Text(oneEvent.description.toString(), 
                    style: TextStyle(fontSize: 20.0),
                    overflow: TextOverflow.ellipsis,),
                ],
              ),
              new Row( 
                children: <Widget>[
                  new Text(oneEvent.place.toString(), 
                    style: TextStyle(color: Colors.grey[350]),
                    overflow: TextOverflow.ellipsis,),
                ],
              ),
              new Row( 
                children: <Widget>[
                  new Text(oneEvent.city.toString() +' - '+ oneEvent.state.toString(), 
                    style: TextStyle(color: Colors.grey[350]),
                    overflow: TextOverflow.ellipsis,),
                ],
              )
            ]
          )          
        ],
      )
    );
  }
}

但每次我尝试显示这些事件时，我都会收到此异常

Exception has occurred.
PlatformException(error, Invalid document reference. Document references must have an even number of segments, but users has 1, null)

我做错了什么？如何在这些表之间进行“内部连接”并显示事件？

我正在使用 Firebase Firestore。

PS：我已经知道 Firestore 是一个 noSQL 数据库并且没有“连接”，但我想做一些类似连接的事情。

Answer 1

正如我在评论中所说，Firestore 不支持多集合查询，因为它没有关系数据库。如果您需要访问多个集合，您将独立管理查询。

这是我通常获取相关集合数据的方式（抱歉，这是 JS 代码，但我不知道 DART）：

    var data = {};

    //First you get users data
    DocumentReference document = Firestore.collection("users")

    document.get().then((snapshot) => {

        //In this case I will store data in some object, so I can add events as an array for a key in each user object

        snapshot.forEach((userDoc) => {
            var userDocData = userDoc.data()

            if (data[userDoc.id] == undefined) {
                data[userDoc.id] = userDocData
            }

        })

        //So in this moment data object contains users, now fill users with events data

//In this var you count how many async events have been downloaded, with results or not.    
var countEvents = 0

        Object.keys(data).forEach((userDocId) => {

    //Here Im creating another query to get all events for each user

            SnapshotReference eventsForCurrentUserRef = Firestore.collection("events").where("userId", "==", userDocId)

            eventsForCurrentUserRef.get.then((eventsForUserSnapshot) => {
//Count events
countEvents++

                eventsForUserSnapshot.forEach((eventDoc) => {

                    var eventDocData = eventDoc.data()

                    //Check if array exists, if not create it
                    if (data[eventDocData.userId].events == undefined) {
                        data[eventDocData.userId].events = []
                    }

                    data[eventDocData.userId].events.push(eventDocData)


                })

if(countEvents == Object.keys(data).length){
//Lookup for events in every user has finished
}

            })


        })

    })