Hibernate Criteria One to Many issues

Question

I am trying to use Hibernate Criteria api to fetch only the topics based on the USER_ID but have no idea how to do it using the criteria.

My Tables are "topic_users" (below)

and "topics" table (below)

I know how to do it using SQL, this would be something like:

SELECT TOPICNAME 
FROM topic_users INNER JOIN topics on topic_users.TOPICS_TOPICS_ID = topics.TOPICS_ID 
WHERE topic_users.USER_ID = 1

This will return all TOPICNAME of USER_ID 1 which is exactly what I want but how I can do this with Hibernate Criteria. So far I have this in my Repository class (see below) but this will only return a highly nested JSON array. I could loop through the objects, use a DTO and build my response or try the Hibernate createSQLQuery method that will let me call a native SQL statement directly (haven't tried that yet)...but I am trying to learn the Criteria so I hope anyone can answer my query.

@Repository("userTopicsDao")
public class UserTopicsDaoImpl extends AbstractDao<Integer, UserTopics>implements UserTopicsDao {

    @Override
    public List<UserTopics> findMyTopics(int userId) {
        Criteria crit = createEntityCriteria();
        crit.add(Restrictions.eq("userId", userId));
        List<UserTopics> userTopicsList = (List<UserTopics>)crit.list();
        return userTopicsList;
    }

and my TOPIC_USERS Entity where I have mapped the TOPICS

@Entity
@Table(name="TOPIC_USERS")
public class UserTopics {

    @Id
    @GeneratedValue(strategy= GenerationType.IDENTITY)
    @Column(name="TOPICUSER_ID")
    private Integer id;

    @Column(name="USER_ID")
    private Integer userId;

    @OneToMany(fetch = FetchType.EAGER)
    @JoinColumn(name = "TOPICS_ID")
    private Set<Topics> topicsUser;

   //getter and setters

Answer 1

Ok starting from the ground up.. you entity classes should look like this:

@Entity
@Table(name="TOPIC_USERS")
public class UserTopics {

    @Id
    @GeneratedValue(strategy= GenerationType.IDENTITY)
    @Column(name="TOPICUSER_ID")
    private Integer id;

    @Column(name="USER_ID")
    private Integer userId;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "TOPICS_TOPICS_ID")
    private Topics topics;

Your Topics class should look like this:

@Entity
@Table(name="TOPICS")
public class Topic {

    @Id
    @GeneratedValue(strategy= GenerationType.IDENTITY)
    @Column(name="TOPICUS_ID")
    private Integer id;

    @Column(name="TOPICNAME")
    private Integer topicName;

    @OneToMany(mappedBy = "topics")
    private Set<UserTopics> userTopics;

Finally the Criteria:

Version 1) You get entire entity:

Criteria c = session.createCriteria(Topics.class, "topics");
c.createAlias("topics.userTopics", "userTopics");
c.add(Restrictions.eq("userTopics.userId", userId));
return c.list(); // here you return List<Topics>

Version 2) You project only the topicname:

Criteria c = session.createCriteria(Topics.class, "topics");
c.createAlias("topics.userTopics", "userTopics");
c.add(Restrictions.eq("userTopics.userId", userId));
c.setProjection(Projections.property("topics.topicName"));
List<Object[]> results =  (List<Object[]>)c.list(); 

// Here you have to manually get the topicname from Object[] table.

}