Fetch only some attributes from table using jpa findall java

Question

I want to select just a few columns from a table.. The catch is that I'm using a specification and pagination from the front end filter, and I don't think I can concatenate those with criteriabuilder. My original idea was to create a @MappedSuperClass with the attributes I wanted (in this case, just the id and date), and fetch using a dao repository from an empty subclass. I have done something similar to this before and it worked, but the subclasses used different tables so it's a different ball game. In this case, since both subclasses use the same table, and there's nothing to differentiate between the classes other than one doesn't have any attributes, it keeps fetching the original bigger class. I want to avoid creating a view with just the columns I want or processing the data in the backend after the fetching, but I think that's the only possible solution.

Superclass

@MappedSupperClass
public class Superclass
{

   @Column( name = "id" )
   private Integer id;

   @Column( name = "date" )
   private Date date;
}

Original Subclass

@Entity
@Table( name = "table" )
public class OriginalSubclass
extends Superclass
{

   @Column( name = "code" )
   private Integer code;

   @Column( name = "name" )
   private String name;
}

New Subclass

@Entity
@Table( name = "table" )
public class NewSubclass
extends Superclass
{
}

I created a new dao for the new subclass

@Repository
public interface NewSubclassDao
extends JpaRepository<NewSubclass, Integer>, JpaSpecificationExecutor<NewSubclass>
{
}

Is there a way to get only the attributes I want with something similar to my idea? Or is it possible to do it with criteriabuilder? If none of the options are viable, would you prefer to use a view or process the data?

EDIT

To make it perfectly clear, I want Spring to bring me only the id and date attributes, using JPA findAll or something very similar, without messing the pagination or filter from the Specification.

Answer 1

You should be able to use @Query to do something like:

@Repository
@Transactional(readOnly = true)
public interface NewSubclassDao
extends JpaRepository<NewSubclass, Integer>, JpaSpecificationExecutor<NewSubclass>
{
    @Query("SELECT table.code FROM #{#entityName} table")              
    public Set<Integer> findAllCodes();
}

Answer 2

There are many ways to do this, but I think this is a perfect use case forBlaze-Persistence Entity Views .

I created the library to allow easy mapping between JPA models and custom interface or abstract class defined models, something like Spring Data Projections on steroids. The idea is that you define your target structure(domain model) the way you like and map attributes(getters) via JPQL expressions to the entity model.

A DTO model for your use case could look like the following with Blaze-Persistence Entity-Views:

@EntityView(User.class)
public interface UserDto {
    @IdMapping
    Long getId();
    String getName();
    Set<RoleDto> getRoles();

    @EntityView(Role.class)
    interface RoleDto {
        @IdMapping
        Long getId();
        String getName();
    }
}

Querying is a matter of applying the entity view to a query, the simplest being just a query by id.

UserDto a = entityViewManager.find(entityManager, UserDto.class, id);

The Spring Data integration allows you to use it almost like Spring Data Projections: https://persistence.blazebit.com/documentation/entity-view/manual/en_US/index.html#spring-data-features

Page<UserDto> findAll(Pageable pageable);

The best part is, it will only fetch the state that is actually necessary!