使用RxJ和AngularFirestore通过外键获取单个文档

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

@Component({
  selector: 'app-example',
  templateUrl: './example.component.html',
  styleUrls: ['./example.component.css']
})
export class ExampleComponent implements OnInit {
  document$: Observable<any>;

  constructor(private firestore: AngularFirestore) { }

  ngOnInit() {
    const foreignKey = 'your_foreign_key_value';

    this.document$ = this.firestore.doc('your_collection/' + foreignKey).snapshotChanges()
      .pipe(
        switchMap(document => {
          if (document.payload.exists) {
            const data = document.payload.data();
            const id = document.payload.id;
            return this.firestore.doc('your_collection/' + id).valueChanges();
          } else {
            // 处理文档不存在的情况
            return null;
          }
        })
      );

    this.document$.subscribe(data => {
      // 处理获取到的文档数据
      console.log(data);
    });
  }
}