This project contains samples of Kotlin-specific features of Spring Data (MongoDB).

Kotlin allows defaulting for constructor- and method arguments. Defaulting allows usage of substitute values if a field in the document is absent or simply null. Spring Data inspects objects whether they are Kotlin types and uses the appropriate constructor.

data class Person (@Id val id: String?, val firstname: String? = "Walter", val lastname: String)

operations.insert<Document>().inCollection("person").one(Document("lastname", "White"))

val walter = operations.findOne<Document>(query(where("lastname").isEqualTo("White")), "person")

assertThat(walter.firstname).isEqualTo("Walter")

Spring Data exposes methods accepting a target type to either query for or to project results values on. Kotlin represents classes with its own type, KClass which can be an obstacle when attempting to obtain a Java Class type.

Spring Data ships with extensions that add overloads for methods accepting a type parameter by either leveraging generics or accepting KClass directly.

operations.getCollectionName<Person>()
 
operations.getCollectionName(Person::class)

Declaring repository interfaces using Kotlin allows expressing nullability constraints on arguments and return types. Spring Data evaluates nullability of arguments and return types and reacts to these. Passing null to a non-nullable argument raises an IllegalArgumentException, as you're already used to from Kotlin. Spring Data helps you also to prevent null in query results. If you wish to return a nullable result, use Kotlin's nullability marker ?. To prevent null results, declare the return type of a query method as non-nullable. In the case a query yields no result, a non-nullable query method throws EmptyResultDataAccessException.

interface PersonRepository : CrudRepository<Person, String> {

	/**
	 * Query method declaring a nullable return type that allows to return null values.
	 */
	fun findOneOrNoneByFirstname(firstname: String): Person?

	/**
	 * Query method declaring a nullable argument.
	 */
	fun findNullableByFirstname(firstname: String?): Person?

	/**
	 * Query method requiring a result. Throws [org.springframework.dao.EmptyResultDataAccessException] if no result is found.
	 */
	fun findOneByFirstname(firstname: String): Person
}