JPQL, Can I Create DTO using Constructor which Takes Entities by Selecting All Columns In Once? - spring

Question

I'm using Spring Data JPA and Hibernate. I made repository having a method returning List<AWithBDto> like
#Query(
"""
select new com.test.example.AWithBDto(a.a1, a.a2, ..., b.b1, b.b2, ...)
from A a join B b where ~
order by ~
"""
)
Because I thought listing all columns of table a and b is tedious, I added a constructor in AWithBDto as follows. (in Kotlin)
data class AWithBDto(
...
) {
constructor(a: A, b: B) : this(
a1 = a.a1,
a2 = a.a2,
...,
b1 = b.b1,
b2 = b.b2,
...,
)
}
So I could shorten my repository codes as follows.
#Query(
"""
select new com.test.example.AWithBDto(a, b)
from A a join B b where ~
order by ~
"""
)
However, the query made kind of N+1 problems because it selected only id columns from both tables.
select
a0_.id as col_0_0_,
b1_.id as col_1_0_
from
a a0_
inner join
b b1_
on a0_.id=b1_.id
where
~
order by
~
and multiple queries selecting all columns for each single row from each table were executed.
select
a0_.id as id1_3_0_,
...,
...,
from
a a0_
where
a0_.id=?
...multiple times
select
b0_.id as id1_4_0_,
...,
...,
from
b b0_
where
b0_.id=?
...multiple times
In JPQL, Can I create DTO using a constructor which takes entities as params by selecting all columns in once? I want Hibernate to execute a single query for this.
Or if I can't, could you suggest good alternatives for me?

Answer 1

I think this is a perfect use case for Blaze-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(A.class)
public interface AWithBDto {
#IdMapping
Long getId();
String getA1();
String getA2();
// Either map properties through nested mapping path expressions
#Mapping("b.b1")
String getB1();
#Mapping("b.b2")
String getB2();
// Or through a subview
BDto getB();
#EntityView(B.class)
interface BDto {
#IdMapping
Long getId();
String getB1();
String getB2();
}
}
Querying is a matter of applying the entity view to a query, the simplest being just a query by id.
AWithBDto a = entityViewManager.find(entityManager, AWithBDto.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<AWithBDto> findAll(Pageable pageable);
The best part is, it will only fetch the state that is actually necessary!