如何使用 snapshotChanges() 方法获取键值并过滤数据？

我正在开发一个 Angular Firebase 项目，我需要过滤数据库并获取键值。目前我正在使用值更改()我的服务代码中的方法（getUnreadBooks 和 getFavouriteBooks 方法内部，如下所示）来获取数据并对其进行过滤。但当我尝试在模板文件中获取键值时，它给我的键值是“未定义”。我试着去快照更改()方法，但无法解决如何使用它来获取键值以及过滤数据。下面是我的 Angular FirebaseService，home.component.ts（我正在其中注入我的服务代码）和home.component.html（模板文件）代码分别为：

import { Injectable } from '@angular/core';
import { AngularFireDatabase } from 'angularfire2/database';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/map';

@Injectable()
export class FirebaseService {

  books: Observable<any[]>;
  unreadBooks;
  favoriteBooks;

  constructor(private db: AngularFireDatabase) {}

  getBooks(){
        this.books = this.db.list('/books').valueChanges() as Observable<any[]>;
        return this.books;
    }       

  getFavoriteBooks(){
    this.favoriteBooks = this.db.list('/books').valueChanges() as Observable<any[]>;
    this.favoriteBooks = this.favoriteBooks.map(books => {
        const topRatedBooks = books.filter(item =>  item.rate>4);
        return topRatedBooks;
    })
    return this.favoriteBooks;
  }

  getUnreadBooks(){
    this.unreadBooks = this.db.list('/books').valueChanges() as Observable<any[]>;
    this.unreadBooks = this.unreadBooks.map(books => {
        const ub = books.filter(item => item.dateread == null);
        return ub;
    })
    return this.unreadBooks;
  }
}

Home.Component.ts 文件 =>

import { Component, OnInit } from '@angular/core';
import { FirebaseService } from '../../services/firebase.service';

@Component({
  selector: 'app-home',
  templateUrl: './home.component.html',
  styleUrls: ['./home.component.css']
})
export class HomeComponent implements OnInit {

  //favorite Books
  favoriteBooks: any;
  unreadBooks: any;

  constructor(private firebaseService: FirebaseService) { }

  ngOnInit() {
    this.firebaseService.getFavoriteBooks()
        .subscribe(favBooks => {
            this.favoriteBooks = favBooks;
            console.log(this.favoriteBooks);
        })
    this.firebaseService.getUnreadBooks()
        .subscribe(ubBooks => {
            this.unreadBooks = ubBooks;
            console.log('Unread Books:', this.unreadBooks);
        })
  }

}

Home.component.html 文件=>

<mat-toolbar>
    My Top Rated Books
</mat-toolbar>
<mat-grid-list cols="3">
    <mat-grid-tile *ngFor="let book of favoriteBooks">
        <mat-card>
            <mat-card-header>
                <mat-card-title>
                    <h4>{{book.title}}</h4>
                </mat-card-title>
            </mat-card-header>
            <img mat-card-image src="{{book.imageUrl}}" alt="{{book.title}}">
            <mat-card-actions>
                <button mat-button mat-raised-button class="detailsButton" [routerLink]="['/book/'+book.$key]">
                    <i class="material-icons">visibility</i>Book Details</button>
                <button mat-button mat-raised-button class="editButton" [routerLink]="['/editbook/'+book.$key]">
                    <i class="material-icons">mode_edit</i>Edit Book</button>
            </mat-card-actions>
        </mat-card>     
    </mat-grid-tile>
</mat-grid-list>

<mat-toolbar>
    Books I have not read yet
</mat-toolbar>
<mat-grid-list cols="3">
    <mat-grid-tile *ngFor="let book of unreadBooks">
        <mat-card>
            <mat-card-header>
                <mat-card-title>
                    <h4>{{book.title}}</h4>
                </mat-card-title>
            </mat-card-header>
            <img mat-card-image src="{{book.imageUrl}}" alt="{{book.title}}">
            <mat-card-actions>
                <button mat-button mat-raised-button class="detailsButton" [routerLink]="['/book/'+book.$key]">
                    <i class="material-icons">visibility</i>Book Details</button>
                <button mat-button mat-raised-button class="editButton" [routerLink]="['/editbook/'+book.$key]">
                    <i class="material-icons">mode_edit</i>Edit Book</button>
            </mat-card-actions>
        </mat-card>     
    </mat-grid-tile>
</mat-grid-list>

声明一个函数来将 id 添加到您的 Object ：

documentToDomainObject = _ => {
    const object = _.payload.doc.data();
    object.id = _.payload.doc.id;
    return object;
}

并在您的 getBooks 方法中使用它：

getBooks(){
  this.books = this.db.list('/books').snapshotChanges()
  .pipe(map(actions => actions.map(this.documentToDomainObject)));

  return this.books;
}