JPA- Joining two tables in non-entity class

Question

I am a newbie ,tried to google but I am unable to solve my query. Please help.

I am trying to map two entities : PersonA and Person in my POJO class PersonC

Answer 1

This post deals with the Hibernate.

The suggestion of putting the @SqlResultSetMapping and @NamedNativeQuery (or @NamedQuery) inside the @Entity class definition is not elegant and evidently does not follow the separation of concerns principle.

The more proper solution is the usage of the @MappedSuperclass annotation as the following:

SingerExtended.java (the class must be abstract):

package pl.music.model.singer.extended;

import javax.persistence.ColumnResult;
import javax.persistence.ConstructorResult;
import javax.persistence.MappedSuperclass;
import javax.persistence.NamedNativeQueries;
import javax.persistence.NamedNativeQuery;
import javax.persistence.SqlResultSetMapping;

@MappedSuperclass
@SqlResultSetMapping( // @formatter:off
    name = "SingerExtendedMapping",
    classes = @ConstructorResult(
        targetClass = SingerExtendedDTO.class,
        columns = {
            @ColumnResult(name = "singer_id", type = Long.class),
            @ColumnResult(name = "first_name"),
            @ColumnResult(name = "last_name"),
            @ColumnResult(name = "count_albums", type = Long.class)
        }
    )
)
@NamedNativeQueries({
    @NamedNativeQuery(
            name = "SingerExtendedAsc",
            query = "select"
                + " singer.singer_id,"
                + " singer.first_name,"
                + " singer.last_name,"
                + " (select count(*) from album where album.singer_id = singer.singer_id) as count_albums"
                + " from singer"
                + " group by singer.singer_id"
                + " order by last_name collate :collation asc, first_name collate :collation asc",
            resultSetMapping = "SingerExtendedMapping"
    )
}) // @formatter:on
public abstract class SingerExtended {
}

then DAO class SingerExtendedDAO.java:

package pl.music.model.singer.extended;

import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.TypedQuery;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class SingerExtendedDAO {

    @PersistenceContext
    EntityManager entityManager;

    @Autowired
    private String collation;

    public List getAll(Integer page, Integer count) {
        TypedQuery query = entityManager.createNamedQuery("SingerExtendedAsc", SingerExtendedDTO.class);
        query.setParameter("collation", collation);
        if ((count != null) && (count.intValue() > 0)) {
            query.setMaxResults(count.intValue());
            if ((page != null) && (page.intValue() >= 0)) {
                query.setFirstResult(count.intValue() * page.intValue());
            }
        }
        List singerExtendedDTOs = query.getResultList();
        return singerExtendedDTOs;
    }

}

and finally the DTO class SingerExtendedDTO.java (you must provide "full" constructor):

package pl.music.model.singer.extended;

public class SingerExtendedDTO {

    private Long singerId;
    private String firstName;
    private String lastName;
    private Long countAlbums;

    // IMPORTANT: this constructor must be defined !!! 
    public SingerExtendedDTO(Long singerId, String firstName, String lastName, Long countAlbums) {
        this.singerId = singerId;
        this.firstName = firstName;
        this.lastName = lastName;
        this.countAlbums = countAlbums;
    }
    ... getters & setters ...
}

If all this is put together the way presented above, we obtain a proper solution:

• everything is in one package,
• query declaration does not pollute any unconcerned entity,
• separation of concerns is preserved (seperated query+mapping, DAO and DTO).