I got a problem with a many to many association in my persistence layer. My scenario is the following:
A user can has several roles and a role can have several user attached to it. During the tests I encountered a strange behavior. I created role object and several user objects. The role was set to each of the users. After this the users were saved using a DAO. Then one of the user gets loaded to check whether he got the role that was passed to him before saving the user object. Calling getRoles() on the user shows that the role was set correctly.
To check whether the inverse direction also works the role object gets loaded from the database using a role DAO. But calling getUsers() on the role object just returns an empty set, although it should contain all the users with this role.
I double checked the database table but everything seems all right. User, role and user_role table were all filled correctly.
So why doesn't the role object contain any user?
I'm using Hibernate and Spring with the following classes.
User class
@Entity
@Table
public class User extends BusinessObject {
...
// Option 1
@ManyToMany(fetch = FetchType.LAZY,
cascade = CascadeType.ALL,
targetEntity=Role.class)
@JoinTable(name= "user_role",
joinColumns = {@JoinColumn(name="user_id")},
inverseJoinColumns = {@JoinColumn(name="role_id")})
// Option 2
@ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
@JoinTable(name= "user_role",
joinColumns = {@JoinColumn(name="user_id")},
inverseJoinColumns = {@JoinColumn(name="role_id")})
private Set<Role> roles = new HashSet<Role>();
...
}
Role class
@Entity
@Table
public class Role extends BusinessObject {
...
// Option 1
@ManyToMany(fetch = FetchType.LAZY,
cascade = CascadeType.ALL,
mappedBy= "roles",
targetEntity = User.class)
// Option 2
@ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
@JoinTable(name= "user_role",
joinColumns = {@JoinColumn(name="role_id")},
inverseJoinColumns = {@JoinColumn(name="user_id")})
private Set<User> users = new HashSet<User>();
...
}
To test I'm using the following code in a JUnit test class.
@Test
public void test(){
Transaction trans = sessionFactory.getCurrentSession().beginTransaction();
Role userAdminRole = new Role();
userAdminRole.setName(RoleName.USER_ADMIN);
Role userRole = new Role();
userRole.setName(RoleName.USER);
User user1 = new User();
user1.setEmail("[email protected]");
user1.getRoles().add(userAdminRole);
user1.getRoles().add(userRole);
userDao.save(user1);
User user2 = new User();
user2.setEmail("[email protected]");
user2.getRoles().add(role);
userDao.save(user2);
User user3 = new User();
user3.setEmail("[email protected]");
user3.getRoles().add(role);
userDao.save(user3);
trans.commit();
User loadedUser = userDao.load(user1.getId());
// Tests passes
Assert.assertNotNull(loadedUser);
Assert.assertEquals(user1, loadedUser);
Set<Role> roles = loadedUser.getRoles();
// Tests passes
Assert.assertEquals(2, roles.size());
Role loadedUserAdminRole = roleDao.findByName(RoleName.USER_ADMIN);
Set<User> users = loadedUserAdminRole.getUsers();
// Test fails: Count is 0 instead of 3 !!!!!!!
Assert.assertEquals(3, users.size());
}
UPDATE
Sorry I forgot to mention one thing. When I tested the code I of course didn't mark the many to many association twice in each class file. Instead I used either option 1 or option 2 in each class file.
The problem probably comes from the fact that you mapped the same bidirectional association twice. If you tell Hibernate twice about the same join table or join column, there is a problem. In a bidirectional association, one of the ends of the association must map the association, and the other one must tell Hibernate that it's the inverse of the other end, using the mappedBy attribute.
Since a many-to-many is completely symmetric, choose one of the end to be the owner (i.e. the end which maps the association, and thus have the @JoinTable annotation). The other side is just the inverse, and thus doesn't have a @JoinTable annotation, but has a mappedBy attribute.
Example:
@Entity
@Table
public class User extends BusinessObject {
...
// This end is the owner of the association
@ManyToMany
@JoinTable(name= "user_role",
joinColumns = {@JoinColumn(name="user_id")},
inverseJoinColumns = {@JoinColumn(name="role_id")})
private Set<Role> roles = new HashSet<Role>();
...
}
@Entity
@Table
public class Role extends BusinessObject {
...
// This end is not the owner. It's the inverse of the User.roles association
@ManyToMany(mappedBy = "roles")
private Set<User> users = new HashSet<User>();
...
}
Additional notes:
Set. It would be useful if the Set was a Set<SomeInterface>mappedBy attribute) to persist the association.Collected from the Internet
Please contact [email protected] to delete if infringement.
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