Spring Data JPA: using object retrieved with `JpaRepository.getOne(id)` - spring

Question

In my Spring Boot application generated with JHipster (v6.0.1) Kotlin blueprint (v0.8.0) I have the following POST request handler
#PostMapping("/book")
fun createBook(#RequestBody createBookVM: CreateBookVM): ResponseEntity<Book> {
val author = authorRepository.getOne(createBookVM.authorId)
val userLogin = SecurityUtils.getCurrentUserLogin().orElseThrow { RuntimeException("User not logged in") }
val user = userRepository.findOneByLogin(userLogin).orElseThrow { RuntimeException("IMPOSSIBLE: user does not exist in DB") }
val book= Book()
book.author = author // FIXME
book.user = user
log.debug("Author object with id : {}", author.id) // THIS WORKS
val result = bookRepository.save(book)
return ResponseEntity.created(URI("/api/books/" + result.id))
.headers(HeaderUtil.createEntityCreationAlert(applicationName, true, ENTITY_NAME, result.id.toString()))
.body(result)
}
The problem is that the author is not added to the book (book.author will be null). However, I can access the values of author as shown with the logging statement. Adding user to book also works fine.
I suppose the problem is that authorRepository.getOne(createBookVM.authorId) returns a proxy object, not an instance of Author, but I do not know how to cope with this situation.

Answer 1

Instead of using authorRepository.getOne(createBookVM.binId) use authorRepository.findById(createBookVM.binId).
T getOne(ID id) returns a reference, not an entity.
/**
* Returns a reference to the entity with the given identifier.
*
* #param id must not be {#literal null}.
* #return a reference to the entity with the given identifier.
* #see EntityManager#getReference(Class, Object)
* #throws javax.persistence.EntityNotFoundException if no entity exists for given {#code id}.
*/
T getOne(ID id);
Optional<T> findById(ID id) returns an entity.
/**
* Retrieves an entity by its id.
*
* #param id must not be {#literal null}.
* #return the entity with the given id or {#literal Optional#empty()} if none found
* #throws IllegalArgumentException if {#code id} is {#literal null}.
*/
Optional<T> findById(ID id);
Also you can use authorRepository.findOne(createBookVM.binId) for older versions than jpa 2.x.x :
/**
* Retrieves an entity by its id.
*
* #param id must not be {#literal null}.
* #return the entity with the given id or {#literal null} if none found
* #throws IllegalArgumentException if {#code id} is {#literal null}
*/
T findOne(ID id);