EJB3 example of adding/removing elements from a ManyToMany relationship

Here's the use case: I have two entities in a many to many relationship, a User and a Referral. A User can have many referrals assigned to them and a Referral can have many Users assigned to it.

Database:
users (User Table)
id (primary key)
username
...

referral (Referral Table)
id (primary key)
name
...

Referral_User_Queue Table (join table)
referral_id (foreign key to the referral table, id column)
user_id (foreign key to the user table, id column)

EJB's

@Entity
@Table(name = "users")
public class User implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
@Column(name = "id", nullable = false)
private Integer id;
...
@JoinTable(name = "referral_user_queue", joinColumns = {@JoinColumn(name = "user_id", referencedColumnName = "id")}, inverseJoinColumns = {@JoinColumn(name = "referral_id", referencedColumnName = "id")})
@ManyToMany
private Collection referralIdCollection;

...
getters and setters
}

@Entity
@Table(name = "referral")
public class Referral implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
@Column(name = "id", nullable = false)
private Integer id;

...

@ManyToMany(mappedBy = "referralIdCollection")
private List userIdCollection;

...
getters and setters
}


The User is the owning entity

I have stateless session beans that are facades to these entities (since netbeans generates them for me). The calls I make below are simply calls that end up as "merge" and "find" nothing more. So a call to userFacade.updateUser(User user) ends up being em.merge(user) where em is the injected EntityManager in the stateless session bean.

Here's the test method (I'm using a servlet as a test client for simplicity)

public class TestReferralServlet extends HttpServlet {

@EJB
private UserFacadeLocal userFacade;

@EJB
private ReferralFacadeLocal referralFacade;

protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
this.moveReferral(out);
.....
}

private void moveReferral(PrintWriter out) {
//locate the referral, I just happen to have one with the
//primary key of 31
Referral referral = referralFacade.find(new Integer(31));

User me = userFacade.findUser(1);
User advisor = userFacade.findUser(3);
User drs = userFacade.findUser(5);
User counslor = userFacade.findUser(6);

out.println("
1) users size=" + referral.getUserIdCollection().size());

//you have to make a copy of the collection or you will get a
//concurrent update exception when removing from within a loop.
List users = new ArrayList(referral.getUserIdCollection().size());
for(User u : referral.getUserIdCollection()) {
users.add(u);
}

//clean out any users and start with a fresh slate
for(User u : users) {
referral.getUserIdCollection().remove(u);
u.getReferralIdCollection().remove(referral);
userFacade.updateUser(u);
referralFacade.updateReferral(referral);
}

//grab fresh copy just to make sure everything is clean
referral = referralFacade.find(new Integer(31));
out.println("
2) users size=" + referral.getUserIdCollection().size());

//add all users to the referral
referral.getUserIdCollection().add(me);
me.getReferralIdCollection().add(referral);

referral.getUserIdCollection().add(drs);
drs.getReferralIdCollection().add(referral);

referral.getUserIdCollection().add(counslor);
counslor.getReferralIdCollection().add(referral);

referral.getUserIdCollection().add(advisor);
advisor.getReferralIdCollection().add(referral);

userFacade.updateUser(me);
userFacade.updateUser(drs);
userFacade.updateUser(counslor);
userFacade.updateUser(advisor);
referralFacade.updateReferral(referral);

//grab fresh copy to make sure everyone was really added
referral = referralFacade.find(new Integer(31));
out.println("
3) users size=" + referral.getUserIdCollection().size());

for(User u : referral.getUserIdCollection()) {
out.println("
" + u.getFirstname());
}

//now remove drs only
drs.getReferralIdCollection().remove(referral);
referral.getUserIdCollection().remove(drs);
userFacade.updateUser(drs);
referralFacade.updateReferral(referral);

//grab fresh copy to make sure only drs was removed
referral = referralFacade.find(new Integer(31));
out.println("
4) users size=" + referral.getUserIdCollection().size());

for(User u : referral.getUserIdCollection()) {
out.println("
" + u.getFirstname());
}

//voila...everything is right in the world and I didn't write any jdbc code
}

}


Just remember to update both ends of the relationship or the cache will think it still has it. That's why I'm calling userFacade.update and referralFacade.update