JPA Entity mapped by SQL Join Query

Question

I want to create an entity ReportEntry which is mapped by an SQL. Here are two tables - user and group .

mysql> desc user;
+------------------------+--------------+------+-----+---------+----------------+
| Field                  | Type         | Null | Key | Default | Extra          |
+------------------------+--------------+------+-----+---------+----------------+
| id                     | int(11)      | NO   | PRI | NULL    | auto_increment |
| firstName              | varchar(255) | YES  |     | NULL    |                |
| lastName               | varchar(255) | YES  |     | NULL    |                |
| active                 | bit(1)       | NO   |     | NULL    |                |
| language               | varchar(20)  | NO   |     | NULL    |                |
| activationDate         | datetime     | YES  |     | NULL    |                |
| group_id               | int(11)      | YES  | MUL | NULL    |                |
|                                                                               |
|                           ...More columns...                                  |
|                                                                               |
+------------------------+--------------+------+-----+---------+----------------+

mysql> desc group;
+----------------------------+--------------+------+-----+---------+----------------+
| Field                      | Type         | Null | Key | Default | Extra          |
+----------------------------+--------------+------+-----+---------+----------------+
| id                         | int(11)      | NO   | PRI | NULL    | auto_increment |
| displayName                | varchar(255) | YES  | MUL | NULL    |                |
| description                | varchar(255) | YES  |     | NULL    |                |
+----------------------------+--------------+------+-----+---------+----------------+

I am joining these tables on group_id using the below SQL query.

SELECT u.* FROM user u JOIN group g ON u.group_id = g.id

For the above query, I want to create an Entity and use the above SQL query as its mapping. How can I achieve this?

I don't want just the mapping, but I also want to use the entity to be able to query for records as well. For example, let's say I am able to create the mapping with entity, RecordEntry , I should be able to get a specific record with query like FROM RecordEntry WHERE id = :id" and I will pass id```` as parameter. That way the final query should get executed in native form would be like(for id=1).

SELECT u.* FROM user u JOIN group g ON u.group_id = g.id AND id = 1

Answer 1

Instead of mapping the result to a POJO, I would suggest creating entities for both classes and mapping them with the one to one annotation JPA provides:

@Entity
public class User{
  @Id
  private long id;

  @OneToOne
  @JoinColumn(name="GROUP_ID")
  private Group group;
  ...
}


@Entity
public class Group{
  @Id
  private long id;
  ...
}

Update to reflect updated question: You could then use eg the Criteria-API (or named queries), to query these entities:

CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
CriteriaQuery criteriaQuery = criteriaBuilder.createQuery();
Root user = criteriaQuery.from(User.class);
criteriaQuery.where(criteriaBuilder.equal(user.get("id"), criteriaBuilder.parameter(Long.class, "id")));
Query query = entityManager.createQuery(criteriaQuery);
query.setParameter("id", id);
User result = (User)query.getSingleResult();

Source: Wikibooks on JPA