Mapping an Observable in array items of another Observable, flattening the result

Question

This title probably needs more explanation.

Basically what I get from the backend is an Observable with an array of racedrivers, and to each of the array items, I want to map another property isOnTrack, which consists of another Observable (simple boolean) I retrieve from the backend. I want to flatten the end result so I don't have an Observable within an Observable. I've tried many of the rxjs operators but I cannot get it to work.

Code that doesn't work:

this.drivers$ = this.db.list('users').valueChanges().pipe(
  map(arr => arr.map( (driver:any) => {
    driver.isOnTrack = this.db.object(`telemetry/${driver.uid}/values/IsOnTrack`).valueChanges();
     return driver
  })),
  mergeAll()
);

This successfully maps the isOnTrack observable to the array items but I can't get it flattened.

Project is on RxJS 6

Update 1

After Jonathan's answer I believe I should have used the word unpacked instead of flattening

The Observable after transformations that I would be looking for should deliver something similar to

of([
  {id: 1, name: 'foo', isOnTrack: true},
  {id: 2, name: 'bar', isOnTrack: true},
  {id: 3, name: 'baz', isOnTrack: false},
])

and after one IsOnTrack is changed in the backend it should emit the complete array again.

of([
  {id: 1, name: 'foo', isOnTrack: false},
  {id: 2, name: 'bar', isOnTrack: true},
  {id: 3, name: 'baz', isOnTrack: false},
])

Answer 1

Mock db functions

// this.db.list('users').valueChanges()
const requestIsOnTrack$ = (id: number): Observable<boolean> => interval(1000).pipe(
  take(3),
  map(() => Math.random() >= 0.5)
)

// this.db.object(`telemetry/${driver.uid}/values/IsOnTrack`).valueChanges()
const requestDrivers$ = () => of([
  {id: 1, name: 'foo'},
  {id: 2, name: 'bar'},
  {id: 3, name: 'baz'},
])

Implementation

const drivers$ = requestDrivers$().pipe(
  map(drivers => drivers.map(driver => requestIsOnTrack$(driver.id).pipe(
    take(1),
    map(isOnTrack => ({
      ...driver,
      isOnTrack
    }))
  ))),
  mergeAll(),
  combineAll()
)

Explanation

The object type living in the observables is just <T> for the ease of not using interfaces

• request all drivers from your db: requestDrivers$() => Observable
• map the request isOnTrack to each driver requestIsOnTrack$(id) => Observable<Observable<T>[]>
• limit your requestOnTrack updates to 1 by using take(1) => Observable<Observable<T>[]>
• map the previous values into the new object ({...driver, isOnTrack}) => Observable<Observable<T>[]>
• mergeAll to split up your array into several emits mergeAll() => Observable<Observable<T>>
• combineAll to unbox the observables and bind all mapped values to an array combineAll() => Observable<T[]>

Here is a running stackblitz