在 Angularfire2 中“加入”Firebase 查询

Question

更新：

我遇到的空值字段问题与我的数据库中不存在的键有关，因此这里的大部分内容都不适用于您的问题。如果您正在寻找一种在 AngularFire2 中“加入”查询的方法，那么下面接受的答案可以很好地解决这个问题。我目前使用combineLatest 而不是forkJoin。为此，您必须import 'rxjs/add/observable/combineLatest';。

我有以下非规范化 Firebase 结构：

members
  -memberid1
    -threads
       -threadid1: true,
       -threadid3: true
    -username: "Adam"
    ...

threads
  -threadid1
      -author: memberid3
      -quality: 67
      -participants
         -memberid2: true,
         -memberid3: true
     ...

我想在我的threads 视图中呈现username，该视图按quality 排序。

我的服务：

getUsername(memberKey: string) {
    return this.af.database.object('/members/' + memberKey + '/username')
}

getFeaturedThreads(): FirebaseListObservable<any[]> {
    return this.af.database.list('/threads', {
        query: {
            orderByChild: 'quality',
            endAt: 10
        }
    });
}

我的组件：

ngOnInit() {
    this.threads = this.featuredThreadsService.getFeaturedThreads()
    this.threads.subscribe( 
        allThreads => 
        allThreads.forEach(thisThread => {
            thisThread.username = this.featuredThreadsService.getUsername(thisThread.author)
            console.log(thisThread.username)
        })
    )
} 

由于某种原因，这会将看起来未完成的 observable 记录到控制台。

我想将这些值放入threads 的属性中，这样我就可以像这样在我的视图中呈现它：

<div *ngFor="let thread of threads | async" class="thread-tile">
    ...
    {{threads.username}}
    ...
</div>

更新：console.log 用于 allThreads 和 thisThread

更新：订阅 getUsername()

this.featuredThreadsService.getUsername(thisThread.author)
        .subscribe( username => console.log(username))

这样做的结果是没有值的对象：

Answer 1

您可以基于 getFeaturedThreads 编写一个 observable，它查询成员并将每个线程的 participants 属性中的值替换为用户名：

import { Observable } from 'rxjs/Observable';
import 'rxjs/add/observable/forkJoin';
import 'rxjs/add/operator/do';
import 'rxjs/add/operator/first';
import 'rxjs/add/operator/switchMap';

let featuredThreadsWithUserNames = this.getFeaturedThreads()

  // Each time the getFeaturedThreads emits, switch to unsubscribe/ignore
  // any pending member queries:

  .switchMap(threads => {

    // Map the threads to the array of observables that are to be
    // joined. When the observables emit a value, update the thread.

    let memberObservables = [];
    threads.forEach(thread => {

      // Add the author:

      memberObservables.push(this.af.database
        .object(`members/${thread.author}`)
        .first()
        .do(value => { thread.author = value.username; })
      );

      // Add the participants:

      Object.keys(thread.participants).forEach(key => {
        memberObservables.push(this.af.database
          .object(`members/${key}`)
          .first()
          .do(value => { thread.participants[key] = value.username; })
        );
      });
    });

    // Join the member observables and use the result selector to
    // return the threads - which will have been updated.

    return Observable.forkJoin(...memberObservables, () => threads);
  });

这将为您提供一个每次 getFeaturedThreads 发射时发射的 observable。但是，如果用户名更改，则不会重新发出。如果这很重要，请将 forkJoin 替换为 combineLatest 并从组合成员 observables 中删除 first 运算符。

Answer 2

为了解决用户连接问题，我编写了一个服务来缓存已经获取的用户，并用最少的代码将它们映射到引用数据中。它使用嵌套的映射结构来进行连接：

constructor(public db: AngularFireDatabase, public users:UserProvider) {
    this.threads = db.list('threads').valueChanges().map(messages => {
      return threads.map((t:Message) => {
        t.user = users.load(t.userid);
        return m;
      });
    });
}

UserProvider 服务如下所示：

@Injectable()
export class UserProvider {
  db: AngularFireDatabase;
  users: Map<String, Observable<User>>;

  constructor(db: AngularFireDatabase) {
    this.db = db;
    this.users = new Map();
  }

  load(userid:string) : Observable<User> {
    if( !this.users.has(userid) ) {
      this.users.set(userid, this.db.object(`members/${userid}`).valueChanges());
    }
    return this.users.get(userid);
  }
}

有一个完整的连接工作示例和所有样板here