✅ Solutions – JPA and Hibernate

This section provides reference solutions and guidance for the assignment.

⚠️ Do NOT read this before attempting the assignment.

---

🧠 Solution Approach

The correct implementation should follow:

Controller → Service → Repository → Database

---

🧱 1. Entity Design

👤 Student Entity

```java id=”stdentity01” @Entity @Table(name = “students”) public class Student {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

private String name;
private String email;

@OneToMany(mappedBy = "student", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
private List<Course> courses;

// getters and setters } ````

---

📘 Course Entity

```java id=”courseentity01” @Entity @Table(name = “courses”) public class Course {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

private String courseName;

@ManyToOne
@JoinColumn(name = "student_id")
private Student student;

// getters and setters } ```

---

🔗 Key Concepts Applied

• mappedBy defines the owning side
• cascade = ALL ensures child objects persist automatically
• FetchType.LAZY improves performance

---

📦 2. Repository Layer

```java id=”repo01” public interface StudentRepository extends JpaRepository<Student, Long> { }

public interface CourseRepository extends JpaRepository<Course, Long> { }

---

# ⚙️ 3. Service Layer

```java id="service01"
@Service
public class StudentService {

    @Autowired
    private StudentRepository studentRepository;

    @Autowired
    private CourseRepository courseRepository;

    public Student createStudent(Student student) {
        return studentRepository.save(student);
    }

    public Course addCourse(Long studentId, Course course) {
        Student student = studentRepository.findById(studentId)
                .orElseThrow(() -> new RuntimeException("Student not found"));

        course.setStudent(student);
        return courseRepository.save(course);
    }

    public List<Student> getAllStudents() {
        return studentRepository.findAll();
    }

    public List<Course> getCoursesByStudent(Long studentId) {
        Student student = studentRepository.findById(studentId)
                .orElseThrow(() -> new RuntimeException("Student not found"));

        return student.getCourses();
    }

    public void deleteStudent(Long id) {
        studentRepository.deleteById(id);
    }
}

---

🌐 4. Controller Layer

```java id=”controller01” @RestController @RequestMapping(“/students”) public class StudentController {

@Autowired
private StudentService service;

@PostMapping
public Student createStudent(@RequestBody Student student) {
    return service.createStudent(student);
}

@GetMapping
public List<Student> getAllStudents() {
    return service.getAllStudents();
}

@DeleteMapping("/{id}")
public void deleteStudent(@PathVariable Long id) {
    service.deleteStudent(id);
}

@PostMapping("/{id}/courses")
public Course addCourse(@PathVariable Long id, @RequestBody Course course) {
    return service.addCourse(id, course);
}

@GetMapping("/{id}/courses")
public List<Course> getCourses(@PathVariable Long id) {
    return service.getCoursesByStudent(id);
} } ```

---

🧪 5. Validation (Improved Version)

```java id=”validation01” @NotBlank private String name;

@Email private String email;

---

# ⚠️ Common Mistakes Explained

## ❌ Missing `mappedBy`

Leads to:

* Extra join table creation
* Incorrect relationship mapping

---

## ❌ Not setting parent in child

```java
course.setStudent(student);

Without this:

• Relationship will not persist correctly

---

❌ Returning Entities Directly

Problem:

• Exposes internal structure
• Causes infinite recursion (in bidirectional mapping)

👉 Solution:

• Use DTOs (advanced)

---

❌ Using EAGER Fetching

Problem:

• Loads unnecessary data
• Impacts performance

👉 Prefer LAZY

---

🔍 Debugging Tips

• Enable SQL logs:

spring.jpa.show-sql=true

• Check:

  • Tables created?
  • Foreign key exists?
  • Queries generated correctly?

---

🚀 Advanced Improvements (Recommended)

• Add DTO layer
• Use ModelMapper or MapStruct
• Add global exception handler
• Add logging
• Add pagination

---

🧠 Final Thoughts

If your solution:

• Uses proper entity relationships
• Follows layered architecture
• Handles data correctly

👉 You are thinking like a backend developer.

---

🏁 Next Step

Proceed to:

👉 Assessment

Evaluate your implementation objectively.

---

🔥 Why this is strong

• Doesn’t overwhelm with full project dump
• Explains why, not just what
• Highlights real mistakes (this is gold for learning)
• Encourages improvement mindset

---