Sync a #ManyToOne relationship after saveAndFlush with spring data jpa - spring-boot

Question

I have this entity in a spring boot project. Having the insertable/updatable false is useful in many cases but causes problems when saving the entity. Here's the code.
#Entity
#Table(name = "BOOK", schema = "DIST")
public class Book {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "ID", nullable = false)
private Integer id;
#ManyToOne
#JoinColumn(name = "AUTHOR_ID", insertable = false, updatable = false)
private Author author;
#Column(name = "AUTHOR_ID")
private Integer authorId;
and this method to save a new book
#Transational
public void test() {
Book book = new Book();
book.setAuthorId(1);
book = bookRepository.saveAndFlush(book);
book.getAuthor().getName(); //author is null
}
The getAuthor() relationship is always null after saving, I'm wondering if there's a way to fetch it. In other words I'd like to be able to force the ManyToOne relationship to be populated but I cannot find a clean way to do it.
This approach doesn't work.
#Transational
public void test() {
Book book = new Book();
book.setAuthorId(1);
book = bookRepository.saveAndFlush(book);
book = bookRepository.findById(book.getId());
book.getAuthor().getName(); //author is still null
}
This one seems to be an options but I don't like it.
#Transational
public void test() {
Book book = new Book();
book.setAuthorId(1);
book = bookRepository.saveAndFlush(book);
entityManager.clear()
book = bookRepository.findById(book.getId());
book.getAuthor().getName(); //author is NOT null
}
Any hints?

Answer 1

I happen to be having and have been investigating how to solve this same issue.
Our data model/use case also requires us to have the foreign key field explicitly defined as a number as well as including the referenced object for being able to easily access its properties.
It seems JPA is expecting us to physically populate the nested object before calling save. Most of the examples on the web only have the many to one field and not a separate numeric ID column field
Adding cache isolation annotation to Book:
#Entity
#Table(name = "BOOK", schema = "DIST")
#Cache(isolation = CacheIsolationType.ISOLATED)
public class Book {
appears to be a solution, in my opinion this is less clunky than explicitly calling EntityManager.clear(). But still requires an explicit getById call after the save. In my scenario I already had the isolation defined for solving other issues so adding the extra get is not a big issue. This code example you gave should work after adding the cache isolation annotation:
#Transational
public void test() {
Book book = new Book();
book.setAuthorId(1);
book = bookRepository.saveAndFlush(book);
book = bookRepository.findById(book.getId());
book.getAuthor().getName();
}
I haven't yet been able to find a better solution. I'll keep an eye on this question to see if anyone provides one.

Answer 2

You can try to refresh the object from the Session of the EntityManager
example:
import javax.persistence.EntityManager;
#Transational
public void test() {
Book book = new Book();
book.setAuthorId(1);
book = bookRepository.saveAndFlush(book);
// this will re-read the state of give object
// from underlying database
entityManager.refresh(book);
book.getAuthor().getName(); //author is NOT null
}