rxjs switchMap and nested observables

Question

I want to ask my firestore for an array of documents which are holding information referencing to other documents.

get "user" document => returns ids of "items" documents => foreach(get "item" document)

getUserOwnedItems(uid:string):Observable<Item[]>{
    return this.firestore
          .collection<any>(`members`, ref => ref.where('uid', '==', uid))
          .valueChanges()
          .pipe(
            switchMap(ids => { // these ids are representing the "item" ids 
              return ids.map(id => {
                return this.firestore
                  .doc<Item>(`items/${id.itemId}`)
                  .valueChanges();
              });
            })
            ,concatAll() // <-- Iam not sure about this
          );
}

The problem is Iam expecting an observable with an Array of Items. Instead I get a observable of Item back.

How can I convert this to

Observable<Item[]>

? Thanks in advance!

My Structure in firestore looks as follows:

members : [
 {
  uid:string,
  itemId:string,
  ...
 }
]

items : [
 {
  id:string,
  ...
 }
]

The Use Case in other words: I want to get all Items related to a specific user identified by its uid.

Update: I found out that all concats mergs or forkJoins are not working in this scenario because the firebase observables will never be completed.

I could do something like ...

return this.firestore
   .doc<Item>(`items/${id.itemId}`)
   .valueChanges().pipe(first());

But this will break my realtime connectivity. Besides this every call of this function will result in reads on FireStore which will cost money.

the idea is to combine all the requests and send them back as one Observable. — Danny
SwitchMap returns a single result, so you actually have a single result. — user4676340

Massimiliano Sartoretto Massimiliano Sartoretto · Accepted Answer · 2019-03-11T10:47:38

You probably wanted to use combineAll instead of concatAll if you need an Observable<Item[]>.

combineAll

Flattens an Observable-of-Observables by applying combineLatest when the Observable-of-Observables completes.

concatAll

Converts a higher-order Observable into a first-order Observable by concatenating the inner Observables in order

rxjs switchMap and nested observables

1 Answers

combineAll

concatAll