类型'Observable<{}>‘上不存在属性'take，update，set’

Question

我试着学习了一个关于Angular + Firebase的教程，但是教程的版本是angular4，我的angular版本是6，我的Firebase和AngularFire2的版本也比教程的最新版本高。

我得到了这个错误

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这是我的源代码

import { Injectable } from '@angular/core';
import { AngularFireDatabase } from '../../node_modules/angularfire2/database';
import { Product } from './models/product';
import { take, map } from 'rxjs/operators'

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

  constructor(private db: AngularFireDatabase) { }

  private create() {
    return this.db.list('/shopping-carts').push({
      dateCreated: new Date().getTime()
    });
  }

  private getCart(cartId: string) {
    return this.db.object('/shopping-carts/' + cartId);
  }

  private async getOrCreateCartId() {
    let cartId = localStorage.getItem('cartId');
    if (cartId) return cartId;

      let result = await this.create();
      localStorage.setItem('cartId', result.key);
      return result.key;
  }

  async addToCart(product: Product) {
    let cartId = await this.getOrCreateCartId();
    let item$ = this.db.object('/shopping-carts/' + cartId + '/items' + product.key).valueChanges();
    item$.take(1).subscribe(item => {
      if (item.$exists()) {
        item$.update({ quantity: item.quantity + 1});
      } else {
        item$.set({ product: product, quantity: 1});
      }
    });

  }
}