Quick question, I got two lists. List A are local photos (Android room database), List B are photos from the network. I want to compare both lists and show a 'favorite' icon if an item from List A is present in List B

create a set with local photo's ids:

val localIds = localPhotos.map { it.id }.toSet()

then just use map on listB

listB.map { it.copy(isFavorite = localIds.contains(it.id)) }

Most likely there is no simplier (with the ~same performance) way to do this

Thnx, will try this approach. To complicate my case a little more, it needs to be reactive. So when a user removes an photo from the local database, this needs to be shown in List B immediately. Both are LiveData lists.

You need something like Rx's combineLatest, in LiveData you need to use MediatorLiveData - implementation will be like this - https://gist.github.com/y-polek/42afe8fdb1518db471fb27e646526a06#file-livedataext-kt-L20

thanks, but I am using coroutines. I will try to return a Flow from my room Dao and in my repository write the logic you provide above.

You said you use livedata.. anyway flow has built-in combineLatest https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/combine-latest.html

LiveData in viewmodels for views to observe, suspending functions in repository and room db. Thnx for the link