concat items from other observable

[Michalzr]

Hello,

I am working on a relatively large application which uses rxjs, and I often find myself needing to "concat" the items from other observable. I can do this in two ways:

observableA$.pipe(
  switchMap(a => combineLatest([ of(a), createObservableB(a) ]))
)

and

observableA$.pipe(
  switchMap(a => createObservableB(a).pipe(map(b => [a, b]))
)

But you can see that it's a bit clumsy.

So my questions are:
1) do you know a better way to achieve the same/similar behavior? I am not familiar with every rxjs operator, so maybe I just missed something..
2) would you say this is a normal use-case, or am I doing something wrong? Would love to hear opinions on this as I'm planning to create a custom operator.

More notes:
The createObservableB is usually a ngrx selector factory, and I am only interested in latest value. For these cases I can simply use already existing concatLatestFrom from ngrx and it works fine.
The issue is when the result of createObservableB(a) is actually asynchronous, for example a from(somePromise). The ngrx concatLatestFrom simply doesn't work in this case and I have to switchMap/mergeMap..

This topic is somewhat related to
#5845
and maybe also
#6974
but not 100%, so I opened a new one, hope you don't mind..

Thanks!