JPA Repository many to one - spring

Question

I have Student entity and Course entity. This is #ManyToOne relationship i.e. Student may attend only one course at a time, but courses may have multiple students.
#Entity
public class Student {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String studentId;
private String firstName;
private String secondName;
#ManyToOne
#JoinColumn(name = "course_id")
//#JsonIgnore
private Course course;
#Entity
public class Course {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
private String courseName;
#OneToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE}, mappedBy = "course", orphanRemoval = true, targetEntity = Student.class)
private List<Student> students = new ArrayList<>();
I post my data with the following json:
{
"id": 1,
"courseName": "course134",
"students" : [
{
"id" : 1,
"studentId": "123",
"firstName": "John1",
"secondName": "Name1"
},
{
"id" : 2,
"studentId": "1234567",
"firstName": "John2",
"secondName": "Name2"
}
then, as I get courses I receive:
{
"id": 1,
"courseName": "course134",
"students": []
}
How to list Students attending specific course?
I made up a Query in StudentRepository
#Query("SELECT s from Student s where s.id = :courseName")
Optional<Student> getStudentByCourseName(String courseName);
Still not working.
this is my Repository code:
#Repository
public interface CourseRepository extends JpaRepository<Course, Long> {
Optional<Course> findCourseByCourseName(String courseName);
#Query("SELECT c.students FROM Course c WHERE c.courseName = :courseName")
Optional<Student> getStudentsByCourseName(String courseName);
}
this is my Service method
public Optional<Student> findStudentByCourse(String courseName){
return courseRepository.getStudentsByCourseName(courseName);
}
and finally my Controller:
#GetMapping("/student/course/{courseName}")
public ResponseEntity<Student> findCoursesWithStudentId(#PathVariable String courseName) {
Optional<Student> byCourseName = studentService.findStudentByCourse(courseName);
if (byCourseName.isPresent()) {
return ResponseEntity.ok(byCourseName.get());
} else {
return ResponseEntity.notFound().build();
}
}

Answer 1

You should query the Course table, not the Student table. Also, the query will return the list, not just one entity, so change your method's return type also...
#Query("SELECT c.students FROM Course c WHERE c.courseName = :courseName")
List<Student> getStudentsByCourseName(String courseName) {}
edit
You can always do it like so:
Excute the simple method:
Course findByCourseName(String courseName) {}
and then just get its Students by a simple:
course.getStudents();