Angular 5火力基地

Question

我正在尝试连接到我的Firebase数据库以访问用户集合，但是不知何故它不起作用。我认为该错误与返回类型有关。

从Firebase获取用户集合的代码：

getAllUsers(): AngularFireList<any[]> {
    return this.db.list('users');
}

从服务获取用户数据的代码：

this.userService.getAllUsers().snapshotChanges().subscribe(data => {
    console.log(data);
});

在控制台中出现以下错误：

zone.js:192 Uncaught TypeError: Object(...) is not a function
    at SwitchMapSubscriber.eval [as project] (changes.js:7)
    at SwitchMapSubscriber._next (switchMap.js:91)
    at SwitchMapSubscriber.Subscriber.next (Subscriber.js:91)
    at RefCountSubscriber.Subscriber._next (Subscriber.js:127)
    at RefCountSubscriber.Subscriber.next (Subscriber.js:91)
    at Subject.next (Subject.js:56)
    at ConnectableSubscriber.Subscriber._next (Subscriber.js:127)
    at ConnectableSubscriber.Subscriber.next (Subscriber.js:91)
    at Notification.observe (Notification.js:32)
    at AsyncAction.DelaySubscriber.dispatch (delay.js:91)