Angular FIrebase 5对象键未显示.所以无法删除

问题已得到解答,但我正在寻找一个,嗯,更直接的问题.看起来奇怪的是,为了能够访问对象键,我们必须实现一个而不是两个映射.

基本firebase db：

可以看出,课程对象显然有键.
加价：

<ul>
  <li *ngFor="let course of courses$| async">
    <b>Key:</b> {{Course.$key}} <!-- doesn't show --!>
    <b>title:</b> {{Course.titlE}}
    <b>Duration:</b> {{Course.Duration}}
    <b>Author:</b> {{Course.Author}}
    <p><button (click)="deleteCourse(coursE)">Remove</button></p>

    <hr> 
  </li>
</ul>

现在,这些课程显示得很好,但我不知道如何获取对该键的引用以便删除它. (或许我在firebaseDatabase对象上没有使用正确的方法).无论哪种方式,当我在控制台中记录密钥时,它显示为未定义.

export class AppComponent {
  courses;
  courses$: AngularFireList<any>;

  constructor(private db: AngularFireDatabasE) {
    this.courses = db.list('/courses');
    this.courses$= this.courses.valueChanges();
  }
  ...
  deleteCourse(coursE) {
    console.log(course.$key); // -> undefined
    this.db.object('/courses/' + course.$key).remove();
  }
}

this.courses$= this.courses.snapshotChanges().map(changes => {
  return changes.map(c => ({ key: c.payload.key,...c.payload.val() }));
});

import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/map';

export class AppComponent {
  courseRef: AngularFireList<any>;
  courses$: Observable<anY[]>;

  constructor(private db: AngularFireDatabasE) {
    this.courseRef = db.list('/courses');
    this.courses$= this.courseRef.snapshotChanges().map(changes => {
        return changes.map(c => ({ key: c.payload.key,...c.payload.val() 
    }));
   });
  }
  ...
  deleteCourse(coursE) {
    console.log(course.key);
    this.db.object('/courses/' + course.key).remove();
  }
}