Observable of array for array of Observables

Question

I have the following situation using Observables in my Angular4 app that I just can't get to work: I want to gather summary data of all my booking days for a overview page. Getting all the days is an Observable, and each day has a list of bookings of that day that I have to retrieve - again an observable source. From this list I calculate a summary of the day. All these summaries I want to emit in a resulting observable.

I have tried lot's of more complicated things, but always the inner observables where not waited on to complete and I got empty summaries. I have gotten back to the basics, and something along these lines should work:

getSummaries(): Observable<BookingDaySummary[]> {
    return this.bookingService.getBookingDays().take(1).mergeMap(
        days => this.calculateSummaryOfDays(days)
    )
};

private calculateSummaryOfDays(days: BookingDay[]): Observable<BookingDaySummary[]> {
    const summaries$ = days.map(day => this.calculateSummary(day));
    // I'm aware that the next line is not correct. 
    // Essentially I want the array of observables to complete 
    // and have an array of the resulting values.
    return Observable.merge(summaries$);
}

private calculateSummary(day: BookingDay): Observable<BookingDaySummary> {
    // ... logic to get summary from one day
}

However, the type of summaries$ is Observable<Observable<BookingDaySummary> and not Observable. So it all boils down to: How do I make an Observable<T[]> from [Observable<T>]?

Also: Should the most inner method I use in .map return an Observable or just be a map on the incoming type to T when intending to produce an Observable<T>?

Answer 1

How do I make an Observable< T[] > from Observable< T >[] ?

forkJoin does this. Using String for your T:

import { of, forkJoin} from 'rxjs';
import { Observable } from 'rxjs'

const source:Array<Observable<String>> = [of('A'), of('B'), of('C') ];

const source2:Observable<String[]> = forkJoin(source);

source2.subscribe((res=>console.log(res)));

https://stackblitz.com/edit/arrayofobs2obsofarray?file=index.ts