Lists & Sets - Mappie

Mappie has extensive support for collection types.

An ObjectMappie defines the functions mapList, mapSet, mapArray (JVM only), and nullable variants mapNullableList, mapNullableSet to map collections directly.

For example, we can use mapList to automatically map a List

object PersonMapper : ObjectMappie<Person, PersonDto>()

val persons: List<Person> = listOf(Person("Sjon"), Person("Piet"))
val personDtos: List<PersonDto> = PersonMapper.mapList(persons)

Mappie also defines several built-in mappers to map collections, most notable for List and Set. These built-in mappers are used to automatically map collection types. When defining a mapping manually, these mappers can be referenced explicitly; they are defined in the package mappie.api.builtin.collections.

For example, suppose we have the data class Book containing a list of Page:

data class Book(val pages: List<Page>)

data class Page(val text: String)

and we have a data class BookDto containing a list of strings

data class BookDto(val pages: List<String>)

We can define a mapping between Book and BookDto by defining two mappers: a mapper for Page to String, which simply gets the text property, and a mapper between Book and BookDto which uses the built-in mapper IterableToListMapper:

object PageMapper : ObjectMappie<Page, String>() {
    override fun map(from: Page): String = from.text
}

object BookMapper : ObjectMappie<Book, BookDto>() {
    override fun map(from: Book): BookDto = mapping {
        BookDto::pages fromProperty Book::pages via IterableToListMapper(PageMapper)
    }
}

Note that in this case BookMapper is superfluous and is equivalent to

object BookMapper : ObjectMappie<Book, BookDto>() {
    override fun map(from: Book): BookDto = mapping {
        BookDto::pages fromProperty Book::pages
    }
}

or even

object BookMapper : ObjectMappie<Book, BookDto>()