How to get a List in one of the fields of DTO JPA Repository - spring-boot

Question

I have a DTO and one of its field is a list. So, there is a main table and another table with one to many relations.
Code:
public class DTO {
id;
List<String> name;
}
public interface Repository extends JpaRepository<Table1, Long> {
#Query("SELECT new abc.com.DTO (" +
" r.id," +
" name"+
" )" +
" FROM Table1 r" +
" join Table2 c on c.xyz.id = r.abc.id"+
" WHERE r.type = :type"
)
List<DTO> find(#Param("type") final String type);
}
say I have in table T1 a row against which we have 3 rows in table 2.
I want dto to get me object obj1 with a list of all 3 names for table2
Currently it returns me 3 separate dtos with 3 rows of table2

Answer 1

As it is stated in the JPA specification (see section 4.8 SELECT Clause):
The SELECT clause can contain one or more of the following elements: an identification variable that ranges over an abstract schema type, a single-valued path expression, a scalar expression, an aggregate expression, a constructor expression.
Note that the SELECT clause must be specified to return only single-valued expressions. So, the query like below is therefore not valid:
SELECT o.lineItems FROM Order AS o

Answer 2

This is not possible with JPA directly as constructor expressions always use flat results. Spring Data Projections should support this, but in an inefficient way i.e. it selects entities and just provides a "view" of these entities through an interface.
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(Table1.class)
public interface DTO {
#IdMapping
Long getId();
#Mapping("Table2[xyz.id = VIEW(abc.id)].name")
Set<String> getNames();
}
Querying is a matter of applying the entity view to a query, the simplest being just a query by id.
DTO a = entityViewManager.find(entityManager, DTO.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
public interface Repository extends JpaRepository<Table1, Long> {
List<DTO> findByType(#Param("type") final String type);
}