Challenge

There is a Many to Many relationship between the course and student tables.

Launch the Starter Project

One to Many

One student can have Many grades. Many grades belong to One student.

The student_id column joins both tables.

One to Many

One course can have Many grades. Many grades belong to One course.

The course_id column joins both tables.

Many to Many

One student can enroll in Many courses. One course can contain Many students.

The course_student table joins both tables.

Harry took four courses.

Hermione took four courses.

Herbology contains three students.

Alchemy contains four students.

The table that joins course and student is called a Join Table.

Task 1

Inside Course, declare a @ManyToMany relationship with a List of students.

    @ManyToMany
    private List<Student> students;

Task 2

Inside Student, declare a @ManyToMany relationship with a List of courses.

Who will own the association?

In the One to Many relationship between student and grade, the foreign key lives inside grade.

As a result, grade owns the association.

@Table(name = "grade")
public class Grade {

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

}

In the One to Many relationship between course and grade, the foreign key column lives inside grade.

As a result, grade owns the association.

@Table(name = "grade")
public class Grade {

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

In a Many to Many relationship, the joining columns live inside of the join table.

So it doesn't matter who owns the association.

Task 3

For the sake of this exercise, Course will own the association. Followed by its @ManyToMany annotation, define a @JoinTable.

1. Set the name equal to course_student

The parameters that follow define the join table's foreign key columns. Each foreign key column references the primary key of one table.

2. Set joinColumns equal to a foreign key column @JoinColumn that references the primary table of the entity owning the association.
3. Set inverseJoinColumns equal to a foreign key column @JoinColumn that references the primary table of the entity that does not own the association.

Expected output

---

The course_student join table appears, but Spring Boot creates another one by default.

Task 4

Place the mappedBy parameter on the non-owning side of the relationship.

Expected output

---

Task 5

In order to avoid a recursive loop, add @JsonIgnore to at least one side of the relationship. For this exercise, add it to the non-owning side.

    @JsonIgnore
    @ManyToMany(mappedBy = "students")
    private List<Course> courses;

Task 6

Inside the CourseServiceImpl, finish writing the code for addStudentToCourse:

    //TODO...
    course.getStudents().add(unwrappedStudent);
    return courseRepository.save(course);

Task 7

Inside the CourseController, finish writing the code for enrollStudentToCourse.

Task 8

From Postman, create a new PUT request under the Course folder.

Make four PUT requests to localhost:8080/course/1/student/(1 to 4).

Postman Output

{
    "id": 1,
    "subject": "Charms",
    "code": "CH104",
    "description": "In this class, you will learn spells concerned with giving an object new and unexpected properties.",
    "students": [
        {
            "id": 2,
            "name": "Ron Weasley",
            "birthDate": "1980-03-01"
        },
        {
            "id": 3,
            "name": "Hermione Granger",
            "birthDate": "1979-09-19"
        },
        {
            "id": 4,
            "name": "Neville Longbottom",
            "birthDate": "1980-07-30"
        },
        {
            "id": 1,
            "name": "Harry Potter",
            "birthDate": "1980-07-31"
        }
    ]
}

H2 Output

Task 9

Currently, you can assign a course duplicate students. Try adding Ron to Charms twice.

In both POJO classes, replace the List collection type with a Set. A Set is a collection type that prevents the addition of duplicate elements. A student cannot enroll in duplicate courses.

private Set<Course> courses;

Similarly, a course cannot contain duplicate students.

private Set<Student> students;

Once again, test your application by trying to duplicate Ron's enrollment in charms.

Task 10

• Inside CourseServiceImpl, finish writing the getEnrolledStudents() method.
• Inside CourseController, finish writing the getEnrolledStudents() handler method.
• Inside StudentServiceImpl, finish writing the getEnrolledCourses() method.
• Inside StudentController, finish writing the getEnrolledCourses() handler method.

Task 11

1. From Postman, create a new GET request under the Student folder.

2. From Postman, create a new GET request under the Course folder.

Task 12

Whenever we query a course, we want to exclude the collection of students from the JSON. Otherwise, the response will be messy and convoluted. Put the @JSONIgnore annotation on the owner side too.

    @JsonIgnore
    @ManyToMany
    @JoinTable(
        name = "course_student",
        joinColumns = @JoinColumn(name = "course_id", referencedColumnName = "id"),
        inverseJoinColumns = @JoinColumn(name = "student_id", referencedColumnName = "id")
    )
    private Set<Student> students;

Task 13

Make 12 PUT requests:

• Enroll Harry in Charms and Potions.
• Enroll Ron in History of Magic and Transfiguration
• Enroll Hermione in Charms, History of Magic, Potions, and Transfiguration
• Enroll Neville in Charms, Potions, and Herbology.

localhost:8080/course/1/student/1
localhost:8080/course/5/student/1

localhost:8080/course/4/student/2
localhost:8080/course/6/student/2

localhost:8080/course/1/student/3
localhost:8080/course/4/student/3
localhost:8080/course/5/student/3
localhost:8080/course/6/student/3


localhost:8080/course/1/student/4
localhost:8080/course/5/student/4
localhost:8080/course/3/student/4

Output

From Postman, read all of the courses that Harry is enrolled in:

[
    {
        "id": 1,
        "subject": "Charms",
        "code": "CH104",
        "description": "In this class, you will learn spells concerned with giving an object new and unexpected properties."
    },
    {
        "id": 5,
        "subject": "Potions",
        "code": "POT102",
        "description": "In this class, you will learn correct mixing and stirring of ingredients to create mixtures with magical effects."
    }
]

From Postman, read all of the courses that Hermione is enrolled in:

[
    {
        "id": 1,
        "subject": "Charms",
        "code": "CH104",
        "description": "In this class, you will learn spells concerned with giving an object new and unexpected properties."
    },
    {
        "id": 4,
        "subject": "History of Magic",
        "code": "HIS393",
        "description": "In this class, you will learn about significant events in wizarding history."
    },
    {
        "id": 6,
        "subject": "Transfiguration",
        "code": "TR442",
        "description": "In this class, you will learn the art of changing the form or appearance of an object."
    },
    {
        "id": 5,
        "subject": "Potions",
        "code": "POT102",
        "description": "In this class, you will learn correct mixing and stirring of ingredients to create mixtures with magical effects."
    }
]

From Postman, read all of the students enrolled in Potions.

[
    {
        "id": 1,
        "name": "Harry Potter",
        "birthDate": "1980-07-31"
    },
    {
        "id": 3,
        "name": "Hermione Granger",
        "birthDate": "1979-09-19"
    },
    {
        "id": 4,
        "name": "Neville Longbottom",
        "birthDate": "1980-07-30"
    }
]

Task 14

Inside GradeServiceImpl.saveGrade(), throw a new StudentNotEnrolledException() if the client assigns a student a grade on a course they are not enrolled in.

    @Override
    public Grade saveGrade(Grade grade, Long studentId, Long courseId) {
        Student student = studentRepository.findById(studentId).get();
        Course course = courseRepository.findById(courseId).get();

        //TODO: if student not enrolled in course, throw StudentNotEnrolledException

        grade.setStudent(student);
        grade.setCourse(course);
        return gradeRepository.save(grade);
    }

Handle the exception by adding StudentNotEnrolledException.class as an argument for the @ExceptionHandler: handleResourceNotFoundException.

Task 15

Enroll Harry in Charms. Then, give him an A+.

{
    "id": 1,
    "score": "A+",
    "student": {
        "id": 1,
        "name": "Harry Potter",
        "birthDate": "1980-07-31"
    },
    "course": {
        "id": 1,
        "subject": "Charms",
        "code": "CH104",
        "description": "In this class, you will learn spells concerned with giving an object new and unexpected properties."
    }
}

Now, give him a grade on a course that he's not enrolled in.

{
    "timestamp": "07-08-2022 01:03:57",
    "message": [
        "The student with id: '1' is not enrolled in the course with id: '2"
    ]
}

Good Luck!