RxExample.kt

data class Category(val id: Int, val name: String, val subCategories: List<SubCategory>)

data class SubCategory(val name: String)

data class Product(val value: Int, val category: SubCategory)

private fun getSubCategories(id: Int) = Observable.just(categories.first { it.id == id }.subCategories)

private fun getProductsByCategories() {
  getSubCategories(categoryId)
  .flatMap { categories -> products.filter { it.category in categories }.toObservable()}
  .subscribe { Log.i("Products: ", it.toString()) }
}