Question

User Entity

class User{
int id;
@OneToMany
Set<Role> roles;
}

:User class have so many other details which i have not written.

DTO

class DTO{
int id;
Set<Role> roles;
DTO(int id, Set<Role> roles){
  this.id = id;
  this.roles= roles;
 }
}

Query

hibernateTemplate.find("select new DTO(u.id, r ) from "+User.class.getName()+ " u inner join u.roles as r");

Issue : throws not a valid constructor found.

With below constructor modification the above query works:

DTO(int id, Role role){
      this.id = id;
     }

Issue: But now it gives multiple DTO records for same user equal to no of roles that user is having. Please help.

Answer 1

Since you need multiple rows to create a single DTO instance, you can’t use the new operator inside the query. Instead, you’ll have to create your DTOs by yourself. Something like this should do:

Map<Long, DTO> dtosById = new LinkedHashMap<Long, DTO>();
List<Object[]> rows = hibernateTemplate.find("select u.id, r from User u inner join u.roles as r");
for (Object[] row : rows) {
    Long id = (Long) row[0];
    Role role = (Role) row[1];
    DTO dto = dtosById.get(id);
    if (dto == null) {
        dto = new DTO(id);
        dtosById.put(id, dto);
    }
    dto.addRole(role);
}
List<DTO> dtos = new ArrayList<DTO>(dtosById.values());