Transform retrieved livedata from different sources (storage, firestore) via Transformations - android-recyclerview

Question

Following situation:
I'm retrieving data from firestore via LiveData to display it in a RecyclerView like this:
val placesDataSet: LiveData<ArrayList<Pair<String, PlaceModel>>> = db.getCollection("places")
PlaceModel has a property previewImageUrl: String which will be passed to Picasso image library inside the RecyclerView.Adapter.
After retrieving the "place" information (which is an ArrayList of Pair<documentId: String, place: PlaceModel I want to get the previewImageUrl from Firebase Storage in the next step and map it to corresponding PlaceModel object before displaying it in the RecyclerView.
The storage data structure is as follows: images/$documentId/preview_image.jpg and therefore I need to get the place information (because of documentId) first to identify the right image folder.
How can I handle this using Transformations? I have also a function called getDownloadUrl(path: String): MutableLiveData<Resource<String>> which returns the downloadUrl for provided path from storage.
What I've tried till now is:
val dataSetFinalState = Transformations.map(placesDataSet) {
mapDownloadUrl(it)
}
private fun mapDownloadUrl(arrayList: ArrayList<Pair<String, PlaceModel>>):
ArrayList<Pair<String, PlaceModel>> {
arrayList.forEach {
it.second.previewImageUrl = // How to call getDownloadUrl(path: String) here and wait for response?
}
return arrayList
}

Answer 1

Unfortionally I couldn't solve this problem with Transformations so I decided to choose another approch. After uploading the preview image I try to get the download url to store this information into database.
The problem with this approch was that I use the image resize extension of firebase with delete original image option. Creating resized images takes a bit time so therefore I had an error in the next step because the image has not been existed. But I solved this problem with retry mechanism.