How to sort by fetched properties with distinct JPA Specifications

Question

I use Spring Boot 1.5.3.RELEASE and for me it's unclear how to sort by properties of nested objects with distinct and Specifications because of:

Caused by: org.postgresql.util.PSQLException: ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list

Spring Data JPA generates wrong query.

Let's see a little example:

Model

@Data
@Entity
@Table(name = "vehicle")
public class Vehicle implements Serializable {

    @Id
    @GeneratedValue(strategy = IDENTITY)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "vehicle_type_id")
    private VehicleType vehicleType;

    @ManyToOne
    @JoinColumn(name = "vehicle_brand_id")
    private VehicleBrand vehicleBrand;
}

We have Vehicle class with nested objects VehicleType and VehicleBrand.

@Data
@Entity
@Table(name = "vehicle_brand")
public class VehicleBrand implements Serializable {

    @Id
    @GeneratedValue(strategy = IDENTITY)
    private Long id;

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

    @ManyToOne
    @JoinColumn(name = "vehicle_model_id")
    private VehicleModel model;

}

Class VehicleBrand also contains VehicleModel.

@Data
@Entity
@Table(name = "vehicle_model")
public class VehicleModel implements Serializable {

    @Id
    @GeneratedValue(strategy = IDENTITY)
    private Long id;

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

Service

Now I want to create a query with JPA Specifications and some sorting by "vehicleBrand.name":

public List<Vehicle> findAll() {
    Specification<Vehicle> spec = Specifications.where(
            (root, criteriaQuery, criteriaBuilder) -> {
                criteriaQuery.distinct(true);
                return null;
            }
    );
    return vehicleRepository.findAll(spec, new Sort("vehicleBrand.name"));
}

Spring Data JPA generates following query:

select
    distinct vehicle0_.id as id1_0_,
    vehicle0_.gas_type as gas_type2_0_,
    vehicle0_.vehicle_brand_id as vehicle_4_0_,
    vehicle0_.vehicle_type_id as vehicle_5_0_,
    vehicle0_.year_of_issue as year_of_3_0_ 
from
    vehicle vehicle0_ 
left outer join
    vehicle_brand vehiclebra1_ 
        on vehicle0_.vehicle_brand_id=vehiclebra1_.id 
order by
    vehiclebra1_.name asc

And it fairly doesn't work because of:

Order by expression "VEHICLEBRA1_.NAME" must be in the result list in this case; SQL statement

To fix the issue we have to fetch vehicleBrand in our Specification:

public List<Vehicle> findAll() {
    Specification<Vehicle> spec = Specifications.where(
            (root, criteriaQuery, criteriaBuilder) -> {
                criteriaQuery.distinct(true);
                root.fetch("vehicleBrand", JoinType.LEFT); //note that JoinType.INNER doesn't work in that case
                return null;
            }
    );
    return vehicleRepository.findAll(spec, new Sort("vehicleBrand.name"));
}

Spring Data JPA generates following query:

select
        distinct vehicle0_.id as id1_0_0_,
        vehiclebra1_.id as id1_1_1_,
        vehicle0_.gas_type as gas_type2_0_0_,
        vehicle0_.vehicle_brand_id as vehicle_4_0_0_,
        vehicle0_.vehicle_type_id as vehicle_5_0_0_,
        vehicle0_.year_of_issue as year_of_3_0_0_,
        vehiclebra1_.vehicle_model_id as vehicle_3_1_1_,
        vehiclebra1_.name as name2_1_1_ 
    from
        vehicle vehicle0_ 
    left outer join
        vehicle_brand vehiclebra1_ 
            on vehicle0_.vehicle_brand_id=vehiclebra1_.id 
    order by
        vehiclebra1_.name asc

And now it works because we see vehiclebra1_.name in the selection part.

Question

But what to do If I need to sort by "vehicleBrand.model.name"?
I make an additional fetch, but it doesn't work:

public List<Vehicle> findAll() {
    Specification<Vehicle> spec = Specifications.where(
            (root, criteriaQuery, criteriaBuilder) -> {
                criteriaQuery.distinct(true);
                root.fetch("vehicleBrand", JoinType.LEFT).fetch("model", JoinType.LEFT);
                return null;
            }
    );
    return vehicleRepository.findAll(spec, new Sort("vehicleBrand.model.name"));
}

It generates following query:

select
        distinct vehicle0_.id as id1_0_0_,
        vehiclebra1_.id as id1_1_1_,
        vehiclemod2_.id as id1_2_2_,
        vehicle0_.gas_type as gas_type2_0_0_,
        vehicle0_.vehicle_brand_id as vehicle_4_0_0_,
        vehicle0_.vehicle_type_id as vehicle_5_0_0_,
        vehicle0_.year_of_issue as year_of_3_0_0_,
        vehiclebra1_.vehicle_model_id as vehicle_3_1_1_,
        vehiclebra1_.name as name2_1_1_,
        vehiclemod2_.name as name2_2_2_ 
    from
        vehicle vehicle0_ 
    left outer join
        vehicle_brand vehiclebra1_ 
            on vehicle0_.vehicle_brand_id=vehiclebra1_.id 
    left outer join
        vehicle_model vehiclemod2_ 
            on vehiclebra1_.vehicle_model_id=vehiclemod2_.id cross 
    join
        vehicle_model vehiclemod4_ 
    where
        vehiclebra1_.vehicle_model_id=vehiclemod4_.id 
    order by
        vehiclemod4_.name asc

And it doesn't work because of:

Order by expression "VEHICLEMOD4_.NAME" must be in the result list in this case; SQL statement

Take a look on how we select vehiclemod2_.name but make order by vehiclemod4_.name.

I've tried to make sorting in Specification directly but it also doesn't work:

Specification<Vehicle> spec = Specifications.where(
        (root, criteriaQuery, criteriaBuilder) -> {
            criteriaQuery.distinct(true);
            root.fetch("vehicleBrand", JoinType.LEFT).fetch("model", JoinType.LEFT);
            criteriaQuery.orderBy(criteriaBuilder.asc(root.join("vehicleBrand", JoinType.LEFT).join("model", JoinType.LEFT).get("name")));
            return null;
        }
);

What should I do to make JPA generate right query so I could make a sorting by nested objects? Does it make sense to upgrade version of Spring Boot from 1.5.3.RELEASE to 2+?
Thanks.

Answer 1

Here's a little secret: you don't need to use the Sort parameter at all.

Just use CriteriaQuery.orderBy:

Specification<Vehicle> spec = Specifications.where(
            (root, criteriaQuery, criteriaBuilder) -> {
                criteriaQuery.distinct(true);
                var model = root.fetch("vehicleBrand", JoinType.LEFT).fetch("model", JoinType.LEFT);
                criteriaQuery.orderBy(criteriaBuilder.asc(model.get("name"));
                return null;
            }
    );
    return vehicleRepository.findAll(spec));

The Sort parameter is likely what's adding the extra join in your scenario.