spring-data/mongodb/kotlin

Spring Data MongoDB - Kotlin examples

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

Value defaulting on entity construction

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")

Kotlin Extensions

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)

Nullability

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
}

Type-Safe Kotlin Mongo Query DSL

Using the Criteria extensions allows to write type-safe queries via an idiomatic API.

operations.find<Person>(Query(Person::firstname isEqualTo "Tyrion"))

Coroutines and Flow support

runBlocking {
	operations.find<Person>(Query(where("firstname").isEqualTo("Tyrion"))).awaitSingle()
}

runBlocking {
	operations.findAll<Person>().asFlow().toList()
}