Firebase RT返回4个对象，但只有3个可用

Question

如果有什么我可以添加来协助请告诉我。尝试获取项目的密钥，但尝试首先解决检索数据并显示它。

app.component.ts

import { Component, OnInit } from '@angular/core';
import { AngularFireDatabase, AngularFireList, AngularFireObject } from 'angularfire2/database';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/map';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})

export class AppComponent {
  coursesRef: AngularFireObject<any>;
  courses: Observable<any>;

  constructor(db: AngularFireDatabase) {
    this.coursesRef = db.object('/courses');
    this.courses = this.coursesRef.valueChanges();

    this.courses.subscribe(course => console.log(course));
  }
}

app.component.html

<ul>
  <li *ngFor="let course of courses | async">
    {{ course | json }}
  </li>
</ul>

这会返回4个对象，尽管数据库只包含3个。

Answer 1

我相信答案是应该在ngOnInit函数中调用该对象。

app.component.ts

import { Component, OnInit } from '@angular/core';
import { AngularFireDatabase, AngularFireList } from 'angularfire2/database';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/map';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})

export class AppComponent implements OnInit {
  coursesRef: AngularFireList<any>;
  courses: Observable<any[]>;

  constructor(private db: AngularFireDatabase) {
    this.coursesRef = db.list('/courses');
  }

  ngOnInit() {
    this.courses = this.coursesRef.valueChanges();

    this.courses.subscribe(course => console.log(course));
  }
}