使用 userId 的 Angular Firestore 应用程序查询不起作用

Question

我有一个带有 firebase 应用程序的 angular 6 我有一个带有可观察对象的服务（我稍后希望多个订阅者收听同一个可观察对象） 还有一个订阅这个 observable 的组件 我试图实现的是获取 auth 对象，从中获取 userId (UID) 并在 db 中搜索具有此 id 的所有文档

组件在 ngOnInit 钩子上调用 observable，但在阶段它仍然为空，所以我得到 ​​p>

无法读取未定义的属性“订阅”

问题是，如果我运行代码来填充订阅块内的 observable（以获取身份验证数据），我看不到数据，如果它在它之外（使用静态 UID 运行同步）比它确实填写正确并显示数据 这是服务的代码

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));
    }
}

组件只是这样做

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

我注意到网上所有的例子都在组件上调用了这个，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 如果我像这样初始化服务

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

所以应用程序当然不会崩溃，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...（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)
    );
}

Answer 1

问题是 expenses$ Observable 将仅在 auth.user$ 发出某些内容后定义，因为那时它被分配了。

解决方案 1 您可以尝试在更高级别组件的构造函数中注入ExpenseService 服务，甚至可能是AppComponent，以便调用ExpenseService 的构造函数并分配expenses$ Observable（您只需要注入它，没有任何方法调用或属性访问）。此时，当您加载组件时，应该定义 Observable

解决方案 2 使用 of 手动创建 Observable expenses$ 然后订阅 this.expensesRef.snapshotChanges() 您可以在 expenses$ 中返回值

类似：

...

expensesSubject = new Subject<Expense[]>();
expenses$: from(expensesSubject);

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.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 };
                    })
                )
            ).subscribe((whatever) => {
              expensesSubject.next(whatever);
            })
        }
    )
}