如何从firestore数据库中获取特定的和有限的检查数据到angular4自动完成输入

import { Component } from '@angular/core';
import { AngularFirestore } from '@angular/fire/firestore';
import { Observable } from 'rxjs';

@Component({
  selector: 'app-your-component',
  templateUrl: './your-component.component.html',
  styleUrls: ['./your-component.component.css']
})
export class YourComponent {
  items: Observable<any[]>;

  constructor(private firestore: AngularFirestore) {
    this.items = this.firestore.collection('your-collection').valueChanges();
  }
}

<input type="text" matInput [matAutocomplete]="auto" [(ngModel)]="selectedItem">
<mat-autocomplete #auto="matAutocomplete">
  <mat-option *ngFor="let item of items | async" [value]="item.name">
    {{ item.name }}
  </mat-option>
</mat-autocomplete>