java - 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

@Entity
class PersonA{
     String sample_field;
}

@Entity
class Person{
     String id;
     String name;

}

Above two are entities of jpa.

Now I want to merge them into one pojo class.

class PersonC
{
   Strind id;
   String address;
}

Tried below code but when I try to fetch Address/Foreign key field it does not work.

@SqlResultSetMapping(name="PersonC", 
classes = {
   @ConstructorResult(targetClass = PersonC.class, 
    columns = {@ColumnResult(name="name")
              , @ColumnResult(name="address")
    )}

where should I define @SqlResultSetMapping ,for which class from the above? ) })

Answer 1

@SqlResultSetMapping can be placed at any entity class (don't annotate POJOs - it won't work). Mapping to POJO class with @ConstructorResult was added in version 2.1 of JPA. POJO used with the mapping has to have correct constructor.

All columns corresponding to arguments of the intended constructor must be specified using the columns element of the ConstructorResult annotation in the same order as that of the argument list of the constructor.

Please consult following example with query usage and work out your case accordingly.

@Entity
public class Address {
    @Id int id;  
    String street;
}


@SqlResultSetMapping(name="PersonDTOMapping",
    classes = {
     @ConstructorResult(targetClass = PersonDTO.class,
       columns = {@ColumnResult(name="name"), @ColumnResult(name="street")}
     )}
)
@Entity
public class Person {
    @Id int id;
    String name;
    Address address;  
}  

public class PersonDTO {
    String name;
    String street;
    public PersonDTO(String name, String street) {
        this.name = name;
        this.street = street;
    }
}

// usage
Query query = em.createNativeQuery(
    "SELECT p.name AS name, a.street AS street FROM Person p, Address a WHERE p.address_id=a.id",
    "PersonDTOMapping");
List<PersonDTO> result = query.getResultList();

Please note that aliases (AS name and AS street) has to match the names in @ColumnResults. The example was tested against Ecliselink 2.5.1.