Spring Boot - relationship deleted on save() method - spring

Question

My problem is this: There is a many to many relationship between two tables - Project and Employee. There is an option to update a given employee, but there is a little problem. After updating the employee, hibernate automatically deletes the employee's record from the connected project_employee table.
Hibernate: update employee set email=?, first_name=?, last_name=? where employee_id=?
And this happens right after that
Hibernate: delete from project_employee where employee_id=?
I'm following a course and I've just noticed this error. Source code of the lecturer is here:
https://github.com/imtiazahmad007/spring-framework-course

Answer 1

I've checked your github page:
#ManyToMany(cascade = {CascadeType.DETACH, CascadeType.MERGE, CascadeType.REFRESH, CascadeType.PERSIST},
fetch = FetchType.LAZY)
#JoinTable(name="project_employee",
joinColumns=#JoinColumn(name="employee_id"),
inverseJoinColumns= #JoinColumn(name="project_id")
)
#JsonIgnore
private List<Project> projects;
CascadeType.MERGE + CascadeType.PERSIST mean, that if Employee entity is saved, Project entity references must be saved.
In may-to-many cases it means:
DELETE by foreign key
Bulk insert
In case there's no bulk insert, there's an issue with persisntence context (your are saving an entity with empty collection of projects).
Possible solutions:
Remove CascadeType.MERGE + CascadeType.PERSIST if you do not want to change projects every time your save Employee. You can still save the ccollection via Repository
Make sure collection is attached on save action. That will cause Delete+Insert, but the resut will be ok.
Change Many-To-Many to One-To-Many with EmbeddedId
Please, refer to documentation:
When an entity is removed from the #ManyToMany collection, Hibernate simply deletes the joining record in the link table. Unfortunately, this operation requires removing all entries associated with a given parent and recreating the ones that are listed in the current running persistent context.
https://docs.jboss.org/hibernate/orm/5.6/userguide/html_single/Hibernate_User_Guide.html#associations-many-to-many
*** Update from dialog below to make cascade clear.
Say, you have two entities A & B (getters and setters omitted). + repos
#Entity
#Table(name = "a")
public class A {
#Id
private Integer id;
private String name;
#ManyToMany(cascade = {CascadeType.ALL})
#JoinTable(name="a_b",
joinColumns=#JoinColumn(name="a_id"),
inverseJoinColumns= #JoinColumn(name="b_id")
)
private List<B> bs;
}
#Entity
#Table(name = "b")
public class B {
#Id
private Integer id;
private String name;
}
You sample test looks like this:
#Test
public void testSave() {
B b = new B();
b.setId(1);
b.setName("b");
b = bRepository.save(b);
A a = new A();
a.setId(1);
a.setName("a");
a.setBs(Collections.singletonList(b));
aRepository.save(a);
a.setName("new");
service.save(a); //watch sevice implementations below
}
Version1:
#Transactional
public void save(A a) {
aRepository.save(a);
}
Hibernate logs are the following:
Hibernate:
update
a
set
name=?
where
id=?
Hibernate:
delete
from
a_b
where
a_id=?
Hibernate:
insert
into
a_b
(a_id, b_id)
values
(?, ?)
delete+bulk insert present (despite the fact, that B-s where not in fact changed)
Version2:
#Transactional
public void save(A a) {
Optional<A> existing = aRepository.findById(a.getId());
if (existing.isPresent()) {
a.setBs(existing.get().getBs());
}
aRepository.save(a);
}
Logs:
update
a
set
name=?
where
id=?
Here b-collection was forcibly re-attached, so hibernate understands, that it's not needed to be cascaded.