Hibernate many-to-many remove relation

Question

I have an issue with an hibernate many-to-many relation: when I remove one item from my set, it is not removed in my database. I know there are tons of similar issues, but I did not succeed in fixing mine by reading them.

I have written a JUnit test case for it. My association is between Buildings and Users:

@Test
public void testBuildingManyToMany(){
    //Create 2 buildings
    Building building = createBuilding("b1");
    Building building2 = createBuilding("b2");
    //Create 1 user
    User user = createUser("u1");

    //Associate the 2 buildings to that user
    user.getBuildings().add(building);
    building.getUsers().add(user);

    user.getBuildings().add(building2);
    building2.getUsers().add(user);

    userController.save(user);
    user = userController.retrieve(user.getId());
    Assert.assertEquals(2, user.getBuildings().size());//Test OK

    //Test 1: remove 1 building from the list
    user.getBuildings().remove(building);
    building.getUsers().remove(user);
    userController.save(user);

    //Test 2: clear and add
    //user.getBuildings().clear();
    //user.getBuildings().add(building);
    //userController.save(user);
    //user = userController.retrieve(user.getId());
    //Assert.assertEquals(1, user.getBuildings().size());
}

Here is the error I got:

...
Hibernate: insert into building_useraccount (userid, buildingid) values (?, ?)
Hibernate: insert into building_useraccount (userid, buildingid) values (?, ?)
Hibernate: delete from building_useraccount where userid=? and buildingid=?
Hibernate: insert into building_useraccount (userid, buildingid) values (?, ?)
4113 [main] WARN org.hibernate.util.JDBCExceptionReporter - SQL Error: 23505, SQLState: 23505
4113 [main] ERROR org.hibernate.util.JDBCExceptionReporter - Unique index or primary key violation: "PRIMARY_KEY_23 ON PUBLIC.BUILDING_USERACCOUNT(BUILDINGID, USERID) VALUES ( /* key:0 */ 201, 201)"; SQL statement:
insert into building_useraccount (userid, buildingid) values (?, ?) [23505-176]

When I comment the "Test 1" and uncomment the "Test 2" lines, I go the following error:

junit.framework.AssertionFailedError: 
Expected :1
Actual   :2

Here are my hbm.xml classes:

<hibernate-mapping default-lazy="true">
    <class name="my.model.pojo.Building" table="building">
    <cache usage="read-write" />
    <id name="id" column="id" type="java.lang.Long">
        <generator class="sequence">
            <param name="sequence">building_id_sequence</param>
        </generator>
    </id>
    <property name="name" type="java.lang.String" column="name" not-null="true" />
    ...
    <set name="users" cascade="none" lazy="true" inverse="true" table="building_useraccount">
        <key column="buildingid" />
        <many-to-many class="my.model.pojo.User" column="userid" />
    </set>
</class>
</hibernate-mapping>

and

<hibernate-mapping default-lazy="true">
<class name="my.model.pojo.User" table="useraccount">
    <cache usage="read-write" />
    <id name="id" column="id" type="java.lang.Long">
        <generator class="sequence">
            <param name="sequence">useraccount_id_sequence</param>
        </generator>
    </id>
    <property name="login" type="java.lang.String" column="login" not-null="true" unique="true" length="40" />

    ...
    <set name="buildings" cascade="none" lazy="false" fetch="join" table="building_useraccount">
        <key column="userid" />
        <many-to-many class="my.model.pojo.Building" column="buildingid" />
    </set>
</class>
</hibernate-mapping>

and the classes

public class User implements Serializable, Identifiable {

private static final long serialVersionUID = 1L;
private int hashCode;

private Long id;
private String login;

private Set<Building> buildings = new HashSet<Building>();

public boolean equals(Object value) {
    if (value == this)
        return true;
    if (value == null || !(value instanceof User))
        return false;
    if (getId() != null && getId().equals(((User) value).getId()))
        return true;
    return super.equals(value);
}

public int hashCode() {
    if (hashCode == 0) {
        hashCode = (getId() == null) ? super.hashCode() : new HashCodeBuilder().append(getId()).toHashCode();
    }
    return hashCode;
}

/* Getter / Setter ... */

and

public class BuildingBase implements Serializable, Identifiable {

private static final long serialVersionUID = 1L;
private int hashCode;

private Long id;
private String name;

private Set<User> users = new HashSet<User>();

public boolean equals(Object value) {
    if (value == this)
        return true;
    if (value == null || !(value instanceof Building))
        return false;
    if (getId() != null && getId().equals(((Building) value).getId()))
        return true;
    return super.equals(value);
}

public int hashCode() {
    if (hashCode == 0) {
        hashCode = (getId() == null) ? super.hashCode() : new HashCodeBuilder().append(getId()).toHashCode();
    }
    return hashCode;
}

/* Getter / Setter ... */

EDIT: Add userController implementation, for the transaction

@Transactional(readOnly = false, propagation = Propagation.REQUIRED)
public User save(User user) throws ServiceException {
    validate(user);//Validation stuffs
    return userDAO.update(user);
}

The userDAO:

public class UserDAOImpl extends HibernateDAOImpl<User> implements UserDAO {
}

And the HibernateDAOImpl:

public class HibernateDAOImpl<T> implements DAO<T> {

    public T update(T entity) {
        return executeAndCreateSessionIfNeeded(new HibernateAction<T>() {
            @Override
            public T execute(Session session) {
                return (T) session.merge(entity);
            }
        });
    }

    protected <E> E executeAndCreateSessionIfNeeded(HibernateAction<E> action) {
        Session session = null;
        try {
            session = sessionFactory.getCurrentSession();
            return executeAction(action, session);
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }

}

Answer 1

The CascadeType.REMOVE doesn't make sense for many-to-many associations because when set on both sides it could trigger a chain deletion between parents and children and back to parents. If you only set it on the parent side, you could bump into issues when a deleting child is still referenced by some other parents.

To quote the Hibernate docs:

It does not usually make sense to enable cascade on a many-to-one or many-to-many association. In fact the @ManyToOne and @ManyToMany don't even offer a orphanRemoval attribute. Cascading is often useful for one-to-one and one-to-many associations.

Answer 2

Why cascade="none"?

You should use cascade="detached,merge,refresh,persist" (not delete !) instead to update removals in collections.

Answer 3

Replacing cascade='none' by cascade='all' on the buildings relationship defined on the user should fix the problem.

Since you are saving the user, in order to also update the many-to-many in the DB, you need to cascade the changes on the relationship from the user.