如何在Firebase和Ionic中从集合中检索一个文档？

Question

我有以下这段代码从一个集合中检索所有文档：

public postCollection: AngularFirestoreCollection<Post>;
public posts: Observable<Post[]>;

constructor(private firestore: AngularFirestore, public afAuth: AngularFireAuth) {
   this.postCollection = firestore.collection<Post>('Posts');
}

ngOnInit() {
    this.posts = this.postCollection.snapshotChanges().pipe(
        map(actions => {
          return actions.map(a => {
            const data = a.payload.doc.data();
            const id = a.payload.doc.id;
            return {id, ...data};
          });
        })
    );
}

那么，现在，我只做一个文件（我知道他的身份证）怎么办？

谢谢！