class BooksViewModel(bookDao: BookDao) : ViewModel() {

    // the LiveData from Room won't be exposed to the view...
    private val dbBooks = bookDao.books()

    // ...because this is what we'll want to expose
    val books = MediatorLiveData<List<Book>>()

    private var currentOrder = ASCENDING

    init {
        // here our MediatorLiveData is basically a proxy to dbBooks
        books.addSource(dbBooks) { result: List<Book>? ->
            result?.let { books.value = sortBooks(it, currentOrder) }
        }
    }

    fun rearrangeBooks(order: BooksOrder) = dbBooks.value?.let {
        // and here we can set the value we want
        books.value = sortBooks(it, order)
    }.also { currentOrder = order }
}