InvalidPipeArgument： '[object Object]' for pipe 'AsyncPipe' at invalidPipeArgumentError

Question

我想从数据库中获取一些数据 这是我的服务

export class CategoryService {
constructor(private db: AngularFireDatabase) { }
getCategories(){
return this.db.list('/categories'); 
}

$Component代码

export class ProductFormComponent implements OnInit {
categories$;
constructor(categoryService: CategoryService) {
this.categories$ = categoryService.getCategories();
}

$here是我的网页

<div class="form-group">
<label for="category">Category</label>  
<select id="category" class="form-control">
<option value=""> </option>
<option *ngFor="let c of categories$ | async" [value]="c.$key">
{{c.name}}
</option>
</select>
</div>

Answer 1

Firebase 库返回承诺。使它们返回可观察量。

import { from } from 'rxjs/operators';
export class CategoryService {
constructor(private db: AngularFireDatabase) { }
getCategories(){
return from(this.db.list('/categories')); 
}

Answer 2

试试这些代码，它应该可以工作

import {Injectable} from '@angular/core';
import {AngularFireDatabase} from '@angular/fire/database';
import {from} from 'rxjs';
@Injectable({
providedIn: 'root'
})
export class CategoryService {
constructor(private db: AngularFireDatabase) {}
getCategories() {
return from(this.db.list('/categories').valueChanges());
}
}