如何使用反应式切换图防止多个网络请求

Question

我正在对我的service.ts进行请求，并将其订阅到组件中。

所有功能都按预期运行，但是这导致对网络的许多请求。我该如何更好地写这个来防止这种情况？

 getDeal(id: string){
    const item: AngularFirestoreCollection<Deal> = this._afs.collection(`deals`,
    (ref) => ref.where(`id`, '==', id));
    return item.valueChanges().pipe(
     // take(1),
      map(arr => {
        return arr;
    }),
      switchMap(arr => {
        const userObservables = arr.map(deal => this._afs.doc(`users/${deal.customerId}`).valueChanges()
          );
         return combineLatest(...userObservables)
            .pipe(
              map((...eusers) => {
                arr.forEach((deal, index) => {
                  deal['customer_username'] =  eusers[0][index].displayName.username;
                  deal['customer_avatar'] = eusers[0][index].photoURL;

              });
              return arr;
            })
            );
      }),
      switchMap(arr => {
        const userObservables = arr.map(deal => this._afs.doc(`users/${deal.dealerId}`).valueChanges()
          );
         return combineLatest(...userObservables)
            .pipe(
              map((...eusers) => {
                arr.forEach((deal, index) => {
                  deal['dealer_username'] =  eusers[0][index].displayName.username;
                  deal['dealer_avatar'] = eusers[0][index].photoURL;
              });

              return arr;
            })
            );
      }),
      share()
  );

  }

在组件中，我进行订阅..

this.subDeal = this._service.getDeal(id).subscribe((deal) => {
console.log(deal) // This should log just once, but it logs multiple times
}):

如何防止多个网络请求？