Hibernate (4.1.2) and Spring (3.1.2) – ManyToMany relationship does not store records in JoinTable - spring

Question

I have a problem and need your help to overcome this issue. Hopefully, this tread may become a reference for similar issues…
In my minimized business model there are Users and Titles. Titles should be created first and can be assigned to many Users, and Users may share the same Titles. Therefore I have created two entities called User and Title with a #ManyToMany relationship and decided that Title should own this relationship. Additionally, I have a UnitTest to run this example.
User Entity
public class User {
Long id;
String name;
Set<Title> titles = new HashSet<Title>();
#Id
#GeneratedValue
#Column(name = "id")
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
#Column(name = "name")
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
/*============ Approach1 ============*/
// #ManyToMany(mappedBy = "users")
/*============ Approach2 ============*/
// #ManyToMany
/*============ Approach3 ============*/
#ManyToMany
#JoinTable( name = "tb_title_user",
joinColumns = #JoinColumn(name = "user_id"),
inverseJoinColumns = #JoinColumn(name = "title_id"))
public Set<Title> getTitles() {
return titles;
}
public void setTitles(Set<Title> titles) {
this.titles = titles;
}
}
Title Entity
public class Title {
Long id;
String description;
Set<User> users = new HashSet<User>();
#Id
#GeneratedValue
#Column(name = "id")
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
#Column(name = "description")
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
/*============ Approach1 & Approach2 & Approach3 ============*/
#ManyToMany
#JoinTable( name = "tb_title_user",
joinColumns = #JoinColumn(name = "title_id"),
inverseJoinColumns = #JoinColumn(name = "user_id"))
public Set<User> getUsers() {
return users;
}
public void setUsers(Set<User> users) {
this.users = users;
}
}
UnitTest
public class UserTest {
#Autowired
private SessionFactory sessionFactory;
#Test
#Rollback(false)
#Transactional
public void saveUser(){
Session session = sessionFactory.getCurrentSession();
String now = new Date().toString();
Title title = new Title();
title.setDescription("TitleDescription: " + now);
session.save(title);
User user = new User();
user.setName("UserName: " + now);
user.getTitles().add(title);
session.saveOrUpdate(user);
}
}
If you look at the code above, you are going to see three different approaches. Below, is described if the data is stored correctly in the database tables:
Title User JoinTable
Approach1 Yes Yes No
Approach2 Yes Yes Yes
Approach3 Yes Yes Yes
Here are my thoughts regarding each approach:
Approach1
According with Hibernate documentation ( http://docs.jboss.org/hibernate/core/4.1/manual/en-US/html/ch07.html#d5e5537 ) I should follow Approach1. Specially, because the documentation explicitly mentions:
“As seen previously, the other side don't have to (must not) describe
the physical mapping: a simple mappedBy argument containing the owner
side property name bind the two.”
If I understood right, I don’t have to (must not) add a #JoinTable in the User entity.
Approach2
It works, but it ignores my #JoinTable definition and creates its own table called: tb_user_tb_title. It smells fishy to me.
Approach3
It works, but the documentation says to do not use it. So, it seems to me that I may regret using this approach in an enterprise product.

Answer 1

The only correct way is the first one:
#ManyToMany(mappedBy = "users")
public Set<Title> getTitles() {
return titles;
}
...
#ManyToMany
#JoinTable(name = "tb_title_user",
joinColumns = #JoinColumn(name = "title_id"),
inverseJoinColumns = #JoinColumn(name = "user_id"))
public Set<User> getUsers() {
return users;
}
The inverse side uses the mappedBy attribute to say: "I'm the inverse side. Go see the users attribute in the target entity to see how this association is mapped."
What you're doing wrong is that you only modify the inverse side in your test. JPA/Hibernate only considers the owner side to know if an association exists. So instead of doing
user.getTitles().add(title);
you should do
title.getUsers().add(user);
or even better, do both, to make sure the object graph is coherent.
I really hope that this tread becomes a reference for similar issues, but I doubt it, because I have already answered this question a gazillion times, and it keeps coming again and again, although it's clearly explained in the documentation:
If the association is bidirectional, one side has to be the owner and one side has to be the inverse end (ie. it will be ignored when updating the relationship values in the association table):
[ follows an example with the appropriate annotations on each side of a bidirectional many-to-namy association ]