Hibernate многие-ко-многим удалить отношение

у меня проблема с отношением hibernate many-to-many: когда я удаляю один элемент из своего набора, он не удаляется в моей базе данных. Я знаю, что есть тонны подобных проблем,но мне не удалось исправить мои, прочитав их.

я написал для него тестовый случай JUnit. Моя связь между зданиями и пользователями:

@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());
}

вот ошибка, которую я получил:

...
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]

когда я комментирую "тест 1" и раскомментирую строки "Тест 2" , я иду следующая ошибка:

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

вот мой hbm.классы xml:

<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>

и

<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>

и классы

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 ... */

и

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: добавить реализацию userController, для транзакции

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

userDAO:

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

и 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();
            }
        }
    }

}