JPA/Hibernate SELECT all parent fields with Child count in Single Query

Question

I am using JPA 2.0, with Hibernate 1.0.1.Final.

I want all Parent table fields with no of children in single query.

In Other word, I want to translate from SQL into CriteriaAPI of JPA/Hibernate.

select kgroup.*, count(userGroup.uid) 
from kernelGroup kgroup 
 left join kernelUserGroup userGroup on (kgroup.groupId = userGroup.groupId) 
group by kgroup.groupId

I have following JPA Entities.

@Entity
@Table(name="kernel_group")
public class KernelGroup implements Serializable {

  @Id
  private int groupId;

  private boolean autoGroup;

  private String groupName;

  @OneToMany
  private Set<KernelUserGroup> kernelUserGroups;

  private long jpaVersion; 
}


@Entity
@Table(name="kernel_usergroup")
public class KernelUserGroup implements Serializable {

  @EmbeddedId
  private KernelUserGroupPK id;

  private long jpaVersion;

  @ManyToOne
  private KernelGroup kernelGroup;

  @ManyToOne
  private KernelUser kernelUser;
}

@Embeddable
public class KernelUserGroupPK implements Serializable {
  private String uid;
  private int groupId;
}

My Current Criteria Query is like this :

CriteriaBuilder cb = em.getCriteriaBuilder();  
CriteriaQuery<KernelGroupDto> cQuery = cb.createQuery(KernelGroupDto.class);  
Root<KernelGroup> root = cQuery.from(KernelGroup.class);  

Join<KernelGroup, KernelUserGroup> userGroupsJoin = root.join(KernelGroup_.kernelUserGroups, JoinType.LEFT);    

cQuery.select(cb.construct(KernelGroupDto.class, root, cb.count(userGroupsJoin.get(KernelUserGroup_.id).get(KernelUserGroupPK_.uid))));

cQuery.groupBy(root.get(KernelGroup_.groupId));    

em.createQuery(cQuery).getResultList();    

Now the Problem is, It fires multiple Queries to the database. 1) One query to retrieve groupId and no of count of users 2) N Queries to retrieve group info for each group.

I want only one Query to retrieve GroupInfo and no of count of the users as shown in Above SQL Query.

Please give me good suggestion.

Answer 1

Implement Using the bellow Code.

CriteriaBuilder cb = em.getCriteriaBuilder();  
CriteriaQuery<KernelGroupDto> cQuery = cb.createQuery(KernelGroupDto.class);  
Root<KernelGroup> root = cQuery.from(KernelGroup.class);  

Join<KernelGroup, KernelUserGroup> userGroupsJoin = 
root.join(KernelGroup_.kernelUserGroups, JoinType.LEFT);    

cQuery.select(cb.construct(KernelGroupDto.class, root. 
<Long>get("id"),cb.count(userGroupsJoin)));
cQuery .groupBy( root.<Long>get("id") );

cQuery.groupBy(root.get(KernelGroup_.groupId));    

em.createQuery(cQuery).getResultList(); 

This code will work. for child count.

you must have the constructor of the class KernelGroupDto(Long id, Long childCount)