错误TypeError：changes.forEach不是函数

Question

尝试从firestore（angular / ionic）获取数据时出现此错误

PizzaProvider.ts

getAllPizzas() {
    return this._afs.collection<Pizzas>('pizzas', ref => ref);
}

比萨饼-list.ts

pizzas: Observable<any[]>;
// Function
this.pizzas = this._pizzaProvider.getAllPizzas().snapshotChanges().pipe(
      map(arr => arr.map(doc => {
        return { id: doc.payload.doc.id, ...doc.payload.doc.data() }
     }))
 );

我收到了错误：

ERROR TypeError: changes.forEach is not a function
 at combineChanges (changes.js:19)
    at ScanSubscriber.accumulator (changes.js:15)
    at ScanSubscriber._tryNext (scan.js:111)
    at ScanSubscriber._next (scan.js:104)
    at ScanSubscriber.Subscriber.next (Subscriber.js:93)
    at MapSubscriber._next (map.js:85)
    at MapSubscriber.Subscriber.next (Subscriber.js:93)
    at MapSubscriber._next (map.js:85)
    at MapSubscriber.Subscriber.next (Subscriber.js:93)
    at RefCountSubscriber.Subscriber._next (Subscriber.js:129)

我做错了什么？