Question

I am trying to persist a a relationship between users and friends with a many to many self join. I am using the following method, but no relation is ever created in the database. The user passed a method parameter is created from user input and only contains an id. It also has a Set of friends with only ids. Here is the code:

 public User saveUserFriends(User user) {

     Transaction transaction = null;
        try {
            User retrievedUser = (User) session.get(User.class, user.getId());

            transaction = session.beginTransaction();

             for(User friend :user.getFriends()){
                System.out.println("friend "+friend.getId());
                friend = (User) session.get(User.class, friend.getId());
                System.out.println("retrieved user id "+user.getId());

                retrievedUser.getFriends().add(friend);

                session.save(retrievedUser);
                transaction.commit();
            }



        }catch (HibernateException e) {
            e.printStackTrace();
        } finally {
            session.close();
        }




    return user;
}

My hibernate mapping looks like this:

<hibernate-mapping>
    <class name="com.User" table="User">
        <id name="id" type="int">
        <column name="userId" />
            <generator class="native" />
        </id>
        <set name="friends" table="User_Friend" 
                inverse="true" lazy="false" cascade="none">
            <key column="userId"/>
            <many-to-many column="friendId" class="com.User" />
        </set>
        <set name="users" table="User_Friend" 
                inverse="true" lazy="false" cascade="none">
            <key column="friendId"/>
            <many-to-many column="userId" class="com.User" />
        </set>
    </class>
</hibernate-mapping>

The output is only selects. Why are the relationships not being created?

Answer 1

I’m not a specialist of XML mapping, but both sides of the many-to-many associations are marked as the inverse side (inverse="true"). So Hibernate doesn’t care about them when persisting the user.