Course has many students and many students belong to one course. How to find to which course a parti-CodePudding

there is a one to many unidirectional mapping from course to students

@Entity
Course {
@OnetoMany
List<Student> students;
}

@Entity
Student {
name;
age;
}

the database schema looks like Course id name duration fee

Student rollno name age course_id

I am using Jpa repository for getting.

Can anyone please tell how to tell in which course a particular student belong to? Please note that i am not using bi-directioal mapping so student entity class has no reference to course.

CodePudding user response：

Your problem is in the database deign. No performant code will rescue you from that fate.

Typically there is a join table between Students and Courses that minimally maintains two keys - the studentId and the courseId.

Need to know the courses for a student? Select from the join table where studentId = ‘X’. Need to know students in a course? Select from the join table where courseId = ‘Y’.

CodePudding user response：

I won't comment on the DB design. But solution for this problem is simple.

Basically you need to implement the relationship correctly.

@Entity
public class Course {
  private Long id;

  @OnetoMany(mappedBy = "course")
  List<Student> students;
}

@Entity
public class Student {
  private Long id;
  private String name;
  private Integer age;

  @ManyToOne
  @JoinColumn(name="course_id")
  private Course course;
}

Then simply use the getter

  Student student = studentRepo.findStudentById(123);
  Course course = student.getCourse();

Here is the full reference: https://www.baeldung.com/hibernate-one-to-many

CodePudding user response：

Your mappings and DB schema are fine, and there is nothing overly special about your mapping situation - it is just a unidirectional mapping, and there is no need for force mapping it bidirectionally and/or fetching the student data if you don't need it.

JPQL: "Select c from Course c join c.students s where s.name = :name"

With spring boot

@Query("Select c from Course c join c.students s where s.name = ?1")
Course findCourseByStudentName(String name);

Your DB setup would need to be looked at to determine if the Course is unique - I'd guess it isn't, in which case you'll want to return a list of courses that might have this student name registered.