如何在Firestore中进行内部联接 [英] How to inner-join in firestore
问题描述
我想构建一个视图以在我的应用程序的列表视图中显示一些事件，例如：
I want to build a view to show some events inside a listview in my app like this:
我有这两个表：
用户
Users
事件
Events
但我不知道该怎么办 USERS 和 EVENTS ...
But I don't know how do a "inner join" between the tables USERS and EVENTS...
我尝试过这个：
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,),
],
)
]
)
],
)
);
}
}
但是每次我尝试显示这些事件时，得到此异常
But every time that I try to show these events I get this exception
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？我如何在这些表之间进行内部联接并显示事件？
What I'm doing wrong? How I can do a "inner join" between thesse tables and show the events?
我正在使用Firebase Firestore。
I'm using the Firebase Firestore.
PS：我已经知道Firestore是一个noSQL数据库，没有联接，但是我想做类似联接的事情。
PS: I already know that Firestore is a noSQL database and have no "joins", but I want to do something like a join.
推荐答案
正如我刚才所说，Firestore不支持多集合查询，因为它没有关系数据库。如果需要访问多个集合，则可以独立管理查询。
As I was telling in the coments Firestore does not support multi collection querys cause its no relational DB. If you need to access multiple collections you would manage querys independently.
这通常是我获取相关集合数据的方式（对不起，这是JS代码，但我不知道DART）：
This is how I usually get related collections data (Sorry this is JS code but I dont know 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
}
})
})
})
这篇关于如何在Firestore中进行内部联接的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
