Angular InvalidPipeArgument:“[object]”用于管道“AsyncPipe”和FIrebase

**错误：管道“AsyncPipe”的InvalidPipeArgument:“[object]”。 **

我正在尝试取消订阅可观察到的

角度cli=6.0.3

HTML home.component.HTML

文档中说您需要使用。值更改：

<div class="starter-template">
  <h1>Firebase Data</h1>
  <ul>
    <li *ngFor="let course of courses$ | async">
      {{ course.$value }}
    </li>
  </ul>
</div>

export class HomeComponent {
  courses$;
  constructor(db: AngularFireDatabase) {
    this.courses$ = db.list('/course');
    }
}

import { Component } from '@angular/core';
import { AngularFireDatabase } from 'angularfire2/database';
import { Observable } from 'rxjs';

@Component({
  selector: 'app-root',
  template: `
  <ul>
    <li *ngFor="let item of items | async">
       {{ item | json }}
    </li>
  </ul>
  `,
})
export class AppComponent {
  items: Observable<any[]>;
  constructor(db: AngularFireDatabase) {
    this.items = db.list('items').valueChanges();
  }
}