使用userId的Angular firestore app查询无法正常工作

我有一个带有firebase应用程序的角度6我有一个带有observable的服务（我后来希望多个订阅者监听这个相同的observable）以及一个订阅这个observable的组件我想要实现的是获取auth对象，得到来自它的userId（UID）并在db中搜索具有此id的所有文档

组件在ngOnInit钩子中调用observable，但在阶段它仍然为null，所以我得到了

无法读取undefined的属性'subscribe'

问题是，如果我运行代码来填充订阅块内的observable（获取auth数据）我不会看到数据，如果它在它之外（使用静态UID运行同步）比它填充正确并在此处显示数据是服务的代码

import { Injectable, EventEmitter } from '@angular/core';
import { AngularFirestore, AngularFirestoreCollection, DocumentChangeAction } from '@angular/fire/firestore';
import { Expense } from '../model/Expense';
import { Observable, ReplaySubject, Subject, from } from 'rxjs';
import { map } from 'rxjs/operators';
import { AuthService } from '../core/services/auth.service';

@Injectable({
    providedIn: 'root',
})
export class ExpenseService {

    private dbPath = '/expenses';

    expenseDetailEvent: EventEmitter<Expense> = new EventEmitter<Expense>();

    expensesRef: AngularFirestoreCollection<Expense> = null;
    expenses$: Observable<Expense[]>;

    constructor(private db: AngularFirestore, auth: AuthService) {
        auth.user$.subscribe(
            user => {
                this.expensesRef = this.db.collection<Expense>(this.dbPath, ref =>
                    ref.where('clientId', '==', user.uid)
                );
                this.expenses$ = this.expensesRef.snapshotChanges().pipe(
                    map(actions =>
                        actions.map(a => {
                            const data = a.payload.doc.data() as Expense;
                            const id = a.payload.doc.id;
                            return { id, ...data };
                        })
                    )
                );
            },
            error => console.log(error)
        );
    }

    createExpense(expense: Expense): Observable<any> {
        return from(this.expensesRef.add(expense));
    }

    updateExpense(expense: Expense): Observable<any> {
        return from(this.expensesRef.doc(expense.id).update(expense));
    }
}

而组件就是这么做的

ngOnInit() {
        this.expenseService.expenses$.subscribe(expenses => this.expenses = expenses);
    }

我注意到所有的例子在线，在组件上调用它，但我觉得数据库调用应该在一个服务中，就像它使用普通的http api调用一样

更新，如果我初始化这样的服务

expenses$: Observable<Expense[]> = of([]);

所以当然应用程序没有崩溃，但我没有看到任何数据，如果我只是将代码拉到外面并使用静态UID而不是它...（当然user.uid与我粘贴的静态字符串相同）

这是更新的代码

constructor(private db: AngularFirestore, auth: AuthService) {
    console.log('being here ,i see the data on screen using the static uid');
    this.expensesRef = this.db.collection<Expense>(this.dbPath, ref =>
        ref.where('clientId', '==', 'noYVScDtKfNB5aFqRkAJR7ghxio2')
    );
    this.expenses$ = this.expensesRef.snapshotChanges().pipe(
        map(actions =>
            actions.map(a => {
                const data = a.payload.doc.data() as Expense;
                const id = a.payload.doc.id;
                return { id, ...data };
            })
        )
    );
    auth.user$.subscribe(
        user => {
            console.log('if inside here than it will not show any data on screen');

        },
        error => console.log(error)
    );
}