类型'Observable <AngularFireAction <DatabaseSnapshot <未知>>>'不能分配给类型'Observable <Course>'

Question

我有一个Angular 8应用程序并使用firebase。我想从服务中检索数据。所以我做了一个检索数据的功能。但是我在该函数上遇到了错误

我在Google上搜索了很多内容，并阅读了Firebase文档。但这并没有解决问题。

这就是我所拥有的功能：

 findCourseByUrl(courseUrl: string): Observable<Course> {

    return this.db.list('courses', ref => {
      return ref.orderByChild('url').equalTo(courseUrl);
    }).snapshotChanges().pipe(map(data => data[0]));

  }

但是我仍然收到此错误：

Type 'Observable<AngularFireAction<DatabaseSnapshot<unknown>>>' is not assignable to type 'Observable<Course>'.
  Type 'AngularFireAction<DatabaseSnapshot<unknown>>' is missing the following properties from type 'Course': id, url

如果我这样做：

  findCourseByUrl(courseUrl: string): Observable<Course> {

    return this.db.list('courses', ref => {
      return ref.orderByChild('url').equalTo(courseUrl);
    }).snapshotChanges().pipe(
        map(actions => {
          return actions.map(a => {
            const data = a.payload.doc.data() as Course;
            return { ...data };
          });
        }));

  }

然后我将收到此错误：

Property 'doc' does not exist on type 'DatabaseSnapshot<unknown>'.
  Property 'doc' does not exist on type 'DatabaseSnapshotExists<unknown>'.ts(2339)

Type 'Observable<{ id: string; url: string; description: string; iconUrl: string; courseListIcon: string; longDescription: string; }[]>' is not assignable to type 'Observable<Course>'.
  Type '{ id: string; url: string; description: string; iconUrl: string; courseListIcon: string; longDescription: string; }[]' is missing the following properties from type 'Course': id, url, description, iconUrl, and 2 more.ts(2322)

这是模型：

export interface Course {
    id: string;
    url: string;
    description: string;
    iconUrl: string;
    courseListIcon: string;
    longDescription: string;
}

Answer 1

尝试像这样更改代码：

findCourseByUrl(courseUrl: string): Observable<Course> {

    return this.db.object('courses', ref => {
      return ref.equalTo(courseUrl);
    }).snapshotChanges().pipe(
        map(actions => {
          return actions.map(a => {
            const data = a.payload.val() as Course;
            const id = a.payload.key;
            return { id, ...data };
          });
        }));
  }

编辑

上面的代码是单个对象和对象列表的混合体，因此不起作用。以下代码用于获取单个对象。

findCourseByUrl(courseUrl: string): Observable<Course> {

    return this.db.object(`courses/${courseUrl}`).snapshotChanges().pipe(
        map(a => {
          const data = a.payload.val() as Course;
          const id = a.payload.key;
          return { id, ...data };
        });
  }