f27645dbb6
Use inherited dependency management for Coroutine dependencies. Slight documentation tweaks. Original pull request: #522.
79 lines
2.9 KiB
Markdown
79 lines
2.9 KiB
Markdown
# Spring Data MongoDB - Kotlin examples
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This project contains samples of Kotlin-specific features of Spring Data (MongoDB).
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## Value defaulting on entity construction
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Kotlin allows defaulting for constructor- and method arguments.
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Defaulting allows usage of substitute values if a field in the document is absent or simply `null`.
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Spring Data inspects objects whether they are Kotlin types and uses the appropriate constructor.
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```kotlin
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data class Person (@Id val id: String?, val firstname: String? = "Walter", val lastname: String)
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operations.insert<Document>().inCollection("person").one(Document("lastname", "White"))
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val walter = operations.findOne<Document>(query(where("lastname").isEqualTo("White")), "person")
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assertThat(walter.firstname).isEqualTo("Walter")
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```
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## Kotlin Extensions
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Spring Data exposes methods accepting a target type to either query for or to project results values on.
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Kotlin represents classes with its own type, `KClass` which can be an obstacle when attempting to obtain a Java `Class` type.
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Spring Data ships with extensions that add overloads for methods accepting a type parameter by either leveraging generics or accepting `KClass` directly.
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```kotlin
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operations.getCollectionName<Person>()
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operations.getCollectionName(Person::class)
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```
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## Nullability
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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`.
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```kotlin
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interface PersonRepository : CrudRepository<Person, String> {
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/**
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* Query method declaring a nullable return type that allows to return null values.
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*/
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fun findOneOrNoneByFirstname(firstname: String): Person?
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/**
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* Query method declaring a nullable argument.
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*/
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fun findNullableByFirstname(firstname: String?): Person?
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/**
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* Query method requiring a result. Throws [org.springframework.dao.EmptyResultDataAccessException] if no result is found.
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*/
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fun findOneByFirstname(firstname: String): Person
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}
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```
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## Type-Safe Kotlin Mongo Query DSL
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Using the `Criteria` extensions allows to write type-safe queries via an idiomatic API.
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```kotlin
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operations.find<Person>(Query(Person::firstname isEqualTo "Tyrion"))
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```
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## Coroutines and Flow support
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```kotlin
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runBlocking {
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operations.find<Person>(Query(where("firstname").isEqualTo("Tyrion"))).awaitSingle()
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}
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```
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```kotlin
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runBlocking {
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operations.findAll<Person>().asFlow().toList()
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}
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```
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