很难用ordeyByChild在Angular中显示我的firebase数据

import { Injectable } from '@angular/core';
import { AngularFireDatabase } from '@angular/fire/database';

@Injectable({
  providedIn: 'root'
})
export class FirebaseService {
  constructor(private db: AngularFireDatabase) {}

  getSortedData() {
    return this.db.list('your-firebase-node').query.orderByChild('child-node').valueChanges();
  }
}

import { Component, OnInit } from '@angular/core';
import { FirebaseService } from 'path-to-your-firebase-service';

@Component({
  selector: 'app-your-component',
  templateUrl: './your-component.component.html',
  styleUrls: ['./your-component.component.css']
})
export class YourComponent implements OnInit {
  sortedData: any[];

  constructor(private firebaseService: FirebaseService) {}

  ngOnInit() {
    this.firebaseService.getSortedData().subscribe(data => {
      this.sortedData = data;
    });
  }
}

<ul>
  <li *ngFor="let item of sortedData">
    {{ item | json }}
  </li>
</ul>