How to iterate from the JPA query in spring boot based on the virtual table such as (the count of students for courses tutorials taken) - spring-boot

Question

I need to retrieve each student with number of courses taken, for example :
count
student
5
John
8
Jenny
And I need the following JSON response from my rest end-point:
[
{
"name_student": John,
"count_course": 5,
},
{
"name_student": Jenny,
"count_course": 8,
},
]
I have made the query to fetch the required data inside the Repository:
#Query(value ="Select count(*), s.student_name from student s INNER JOIN courses c on c.student_id= s.student_id group by s.student_name", nativeQuery = true )
List<StudentDTO> getIncidentCountByOfficerKPI ()
My DTO:
#Data
#NoArgsConstructor
public class StudentDTO {
Integer count;
String StudentName;
}
Service:
public List<StudentDTO> getStudentCount() {
return p2StudentRepository.getIncidentCountByOfficerKPI ();
}
Controller:
#GetMapping("/Student-count")
public ResponseEntity<StudentDTO> getAllStudentCount() {
List<StudentDTO> selectedCount = p2StudentService.getStudentCount();
List<StudentDTO> result = new ArrayList<>();
selectedCount.forEach(i -> {
StudentDTOitem = new StudentDTO();
item.setUserName(i.getStudentName());
item.setCount(i.getCount());
result.add(item);
});
return ResponseEntity.ok(result);
}
when I test with postman, it returns to nothing like below
[]
How can I fix that?

Answer 1

The return type of your rest end-point is incorrect, it should be ResponseEntity<List<StudentDTO>> (since you need to place multiple StudentDTO in the response).
And to provide ResponseEntity with a response body, you can use body() method of the ResponseEntity.BodyBuilder (that's a builder-object produced by the static method ResponseEntity.ok()), or it can be done by using of the parameterized constructors of the ResponseEntity.
And since you're actually not transforming the data returned by getStudentCount() you can attach this list as the response body directly.
#GetMapping("/Student-count")
public ResponseEntity<List<StudentDTO>> getAllStudentCount() { // <- change the return type
List<StudentDTO> selectedCount = p2StudentService.getStudentCount();
return ResponseEntity.ok()
.body(selectedCount); // <- add the response body
}