JPA returns multiple objects of the same instance when listing all entities of a class - jpa

Question

I have a JPA entity with a list of child entities. In this case a user entity with roles attached to it.
It looks (a bit simplified - some fields/methods omitted) like this:
#Entity
public class MyUser{
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long myUserId;
private String username;
#OneToMany
#JoinTable(name = "userrole",
joinColumns = {
#JoinColumn(name="myUserId", unique = true)
},
inverseJoinColumns = {
#JoinColumn(name="roleId")
}
)
private Collection<Role> roles;
public Collection<Role> getRoles() {
return roles;
}
}
If intressting, the Role entity is very simple.
#Entity
public class Role {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long roleId;
private String role; // a few more string fields here .
When I add two users and a few hundred roles per user I get a wierd behaviour when I list the users. Each user get's listed a few hundred times (same user = same unique id).
The problematic code:
Query q = em.createQuery("SELECT u FROM MyUser u LEFT JOIN FETCH u.roles");
Collection<MyUser> users = q.getResultList();
for(MyUser u : users){
// print/use u here
}
However, when I just access the database and do select statements, it seems fine. Every user exists only once.
I use OpenJPA 1.2 together with a IBM DB2 database in this case.

Answer 1

I think you have your model wrong, typically a user-role relationship is not OneToMany but "ManyToMany" so you should change your code to look something like this:
#Entity
public class MyUser{
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long myUserId;
private String username;
#ManyToMany //This should be many to many
#JoinTable(name = "userrole",
joinColumns = {
#JoinColumn(name="myUserId") //The userId in the join table should
//NOT be unique because the userId can
//be many times with different roles
},
inverseJoinColumns = {
#JoinColumn(name="roleId")
}
)
private Collection<Role> roles;
public Collection<Role> getRoles() {
return roles;
}
}
Try this way and see if it works.
Also your query shouldn't need the Left Join, the roles should be fetched automatically by JPA once you use the getRoles() method on each entity (using LAZY Fetch)

Answer 2

Actually, it's reasonable to have #ManyToMany mapping for User and UserRole entities. The problem with your query is that it returns all the rows from the join table what I believe you don't need. So just add group by u to your query as follows:
SELECT u FROM MyUser u LEFT JOIN FETCH u.roles GROUP BY u
and you'll be done.