Mapping many-to-many association table with extra column(s)

Question

My database contains 3 tables: User and Service entities have many-to-many relationship and are joined with the SERVICE_USER table as follows:

USERS - SERVICE_USER - SERVICES

SERVICE_USER table contains additional BLOCKED column.

What is the best way to perform such a mapping? These are my Entity classes

@Entity
@Table(name = "USERS")
public class User implements java.io.Serializable {

private String userid;
private String email;

@Id
@Column(name = "USERID", unique = true, nullable = false,)
public String getUserid() {
return this.userid;
}

.... some get/set methods
}

@Entity
@Table(name = "SERVICES")
public class CmsService implements java.io.Serializable {
private String serviceCode;

@Id
@Column(name = "SERVICE_CODE", unique = true, nullable = false, length = 100)
public String getServiceCode() {
return this.serviceCode;
}
.... some additional fields and get/set methods
}

I followed this example http://giannigar.wordpress.com/2009/09/04/m ... using-jpa/ Here is some test code:

User user = new User();
user.setEmail("e2");
user.setUserid("ui2");
user.setPassword("p2");

CmsService service= new CmsService("cd2","name2");

List<UserService> userServiceList = new ArrayList<UserService>();

UserService userService = new UserService();
userService.setService(service);
userService.setUser(user);
userService.setBlocked(true);
service.getUserServices().add(userService);

userDAO.save(user);

The problem is that hibernate persists User object and UserService one. No success with the CmsService object

I tried to use EAGER fetch - no progress

Is it possible to achieve the behaviour I'm expecting with the mapping provided above?

Maybe there is some more elegant way of mapping many to many join table with additional column?

JB Nizet JB Nizet · Accepted Answer · 2011-02-26T13:51:16

Since the SERVICE_USER table is not a pure join table, but has additional functional fields (blocked), you must map it as an entity, and decompose the many to many association between User and Service into two OneToMany associations : One User has many UserServices, and one Service has many UserServices.

You haven't shown us the most important part : the mapping and initialization of the relationships between your entities (i.e. the part you have problems with). So I'll show you how it should look like.

If you make the relationships bidirectional, you should thus have

class User {
    @OneToMany(mappedBy = "user")
    private Set<UserService> userServices = new HashSet<UserService>();
}

class UserService {
    @ManyToOne
    @JoinColumn(name = "user_id")
    private User user;

    @ManyToOne
    @JoinColumn(name = "service_code")
    private Service service;

    @Column(name = "blocked")
    private boolean blocked;
}

class Service {
    @OneToMany(mappedBy = "service")
    private Set<UserService> userServices = new HashSet<UserService>();
}

If you don't put any cascade on your relationships, then you must persist/save all the entities. Although only the owning side of the relationship (here, the UserService side) must be initialized, it's also a good practice to make sure both sides are in coherence.

User user = new User();
Service service = new Service();
UserService userService = new UserService();

user.addUserService(userService);
userService.setUser(user);

service.addUserService(userService);
userService.setService(service);

session.save(user);
session.save(service);
session.save(userService);

Mapping many-to-many association table with extra column(s)

3 Answers