在进行分页查询时，startAfter应在查询之前设置。以下是正确使用startAfter的示例代码：

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

@Component({
  selector: 'app-posts',
  template: `
    

      {{ post.title }}
    
    Load More
  `,
})
export class PostsComponent {
  private batchSize = 5;
  private lastVisible: any = null;

  posts: Observable;

  constructor(private afs: AngularFirestore) {
    this.posts = this.afs
      .collection('posts', ref => ref.orderBy('created', 'desc').limit(this.batchSize))
      .valueChanges();

  loadMore() {
    this.afs
      .collection('posts', ref => ref.orderBy('created', 'desc').startAfter(this.lastVisible).limit(this.batchSize))
      .valueChanges({idField: 'id'})
      .subscribe((posts: any[]) => {
        if (posts.length > 0) {
          this.lastVisible = posts[posts.length - 1].id;
          this.posts = this.posts.pipe(map(arr => arr.concat(posts)));
        }
      });
  }
}