Method to add new Subject

[left sail]

My method addSubject()'s goal is to add a new subject to db, first it checks whether a particular student exist only then it proceeds to save the subject otherwise it throws a user defined exception StudentDoesNotExistException

[raw moth]

<@&987246883653156906> please have a look, thanks.

[raw moth]

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[left sail]

  public void addSubject(Integer id, Subjects subject) {
        Optional<Student> studentOptional = studentRepository.findById(id);

        studentOptional.ifPresentOrElse(student -> {subjectRepository.save(subject);}, ()-> {
            try {
                throw new StudentDoestNotExistException("Student is not found");
            } catch (StudentDoestNotExistException e) {
                throw new RuntimeException(e);
            }
        });
    }```

im wondering if this is the correct way to do it

[solar compass]

yea looks good

[heavy flower]

why not changing this {subjectRepository.save(subject);} to this subjectRepository.save(subject)

also why this:

            try {
                throw new StudentDoestNotExistException("Student is not found");
            } catch (StudentDoestNotExistException e) {
                throw new RuntimeException(e);
            }

[left sail]

kinda new with lambda expressions i tried to do best i could, ill recheck and correct it😅

[left sail]

intellij proposed to add try/catch i just wanted to throw the exception

the second expression was supposed to be just the throw statement in ifPresentOrElse

[heavy flower]

not sure but I think you can change it even to this:
student -> {subjectRepository.save(subject);}
to
subjectRepository::save

[left sail]

what's this called btw i remember seeing it somewhere

part of java 8?

[heavy flower]

its called method references

[left sail]

ah oki thanks for sharing ill read into it

is it supposed to be an alternative to lambdas?

[foggy prawn]

"yes"

its more readable in many situations and technically also implemented more efficiently

[left sail]

lambdas def look confusing to me

ill try to use it instead

[foggy prawn]

its just a very short way of implementing a method of a functional interface

and method references are even shorter

[left sail]

yes it's just too short prolly makes sense a lot when you use it often

[foggy prawn]

yeah. u have to become familiar with it

[left sail]

btw when using ifPresentOrElse, i wanna throw an exception and squid pointed out it might not be correct way of doing this

[foggy prawn]

"if present or else throw" is the same as orElseThrow()

[left sail]

but i only pass in the else part in there?

let me look at orElseThrow

[foggy prawn]

String name = maybePerson.orElseThrow().getName()

[left sail]

oh when maybePerson is valid it accepts it

otherwise calls getName()

[foggy prawn]

orElseThrow() either throws or returns the contained object if its present

its a getOrElseThrow(), essentially

so if u want to use getName() if its there and throw if its not there, u write what i wrote above

so in ur case that would look like:

Student student = studentRepository.findById(id)
  .orElseThrow(() -> new StudentDoesNotExistException(id));

subjectRepository.save(subject); // use student somewhere? lol

regarding the exception, make it an unchecked exception and give it a constructor to accept the ID

[left sail]

ah oki

actually i don't want to add subject if a student doest not exist

that's why the check

[foggy prawn]

it doesnt

it throws if the student wasnt found

so the method is aborted

exceptions stop the flow

[left sail]

oh no i thought you meant in comments //use student somewhere?

[foggy prawn]

ah okay

then the variable is obsolete

but yeah

[left sail]

would it be more complicated if i wanna check if the sub already exists in there

[foggy prawn]

and i would actually use existsById instead

if (!studentRepository.existsById(id)) {
  throw new StudentNotFoundException(id);
}

subjectRepository.save(subject);

(no need to retrieve the object and use optional if u dont even intent on using the student)

[left sail]

this looks a lot cleaner

oki got it use optionals when intend to use the obj

[foggy prawn]

👌

[left sail]

if i wanna make another check if sub exists or not then it would be a another nested if statement?

to avoid adding duplicate subjects

[foggy prawn]

whats the ID on that repository?

[left sail]

it's student id

each student a few subjects

[foggy prawn]

it would just overwrite it then

wouldnt it?

like, if u have a subject with the same student ID, it would silently overwrite the old entry

[left sail]

ye maybe i could get all the subjects and iterate over it?

[foggy prawn]

what do u want to do if there is already a subject with that ID?

throw? or just silently not do anything?

[left sail]

since im think a comparison by subject name would be more fitting

if a subject with same name exists, then i'd throw a user def exception

[foggy prawn]

why subject with the same name and not work on its ID?

multiple students could have the same subject, couldnt they

so there have to be multiple "Math" entries

but with different student IDs

[left sail]

subject table would have multiple math entries true

but the fk is student id

[foggy prawn]

the ID on that repo, u said

[left sail]

we can grab all subjects of a particular stud

[left sail]

there's two tables

[foggy prawn]

yeah but subjectRepository has an ID as well

[left sail]

yes

subID

[foggy prawn]

ah

[left sail]

which is unique for each entry

[foggy prawn]

i see

so u dont want a new math entry in that table

but its not possible to have 2 Math objects with different IDs, is it

the ID is part of the subject object

so when u do save, it looks under the subject-ID

[left sail]

two math subjects can not have say same studID which is fk for the table

[foggy prawn]

yeah but subject-ID

its the same

[left sail]

wait isnt that unique for each entry, auto generated

[foggy prawn]

yeah but its generated when u create ur object

and at this point in ur method u already have it

Subject subject

so im thinking: whats the problem with doing studentRepository.save(subject) again

it wouldnt change anything, would it

[left sail]

a student could add same subject, since im not passing in subID along with sub obj so it would basically add duplicate subjects for same student if i want to add

[foggy prawn]

i dont fully follow. have u tried it? if so, can u show a screenshot of the database entries so that we can see the problem

[left sail]

i havent tried that part yet, ill give it a go and see if it creates a problem

thanks for help

[left sail]

update: changed the addSubject method as zabu suggested, when i try to add say a new subject with a same name it adds it to db

this is my subject table after calling the endpoint

updated addSubject method

pls do ping

[foggy prawn]

and u dont want to add it again, even if the remaining values are totally different?

or overwrite the existing?

[left sail]

each subject is unique to each student

as you can see both maths subs have same studId, basically when say student with id 1 tried to add maths multiple times

[foggy prawn]

yeah but those 2 entries are different

5, 3 vs 7, 3

so which one do u want

old or new

[left sail]

oh if i keep it same do you think it wont add it?

[foggy prawn]

my question is: what do u want to achieve

u showed ur current DB

but what would be the expected DB

[left sail]

well it shouldnt add the subject and throw a exception because subject with such name already exists

[foggy prawn]

okay

is there a reason u have the subId if u dont use it at all here?

like, why do both math objects have different IDs. that shouldnt be possible in the first place

is the ID part of the subject class or not?

u definitely have to index the subject name if u need to use it

[left sail]

i don't really have a need for subID, i assumed each table has its own unique index so i included it

[foggy prawn]

how do u even get to a place where u end up having multiple math objects (despite the same student ID)

[left sail]

are you asking when this would happen?

 
package com.studentportal.student.subject;

import com.studentportal.student.Student;

import javax.persistence.*;

@Entity
public class Subjects {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name="subid")
    private Integer subId;

    @Column(name="subname")
    private String subName;
    @Column(name="totalclasses")
    private int totalClasses;
    @Column(name="classesattended")
    private int classesAttended;
    //private Integer studentId;

    @ManyToOne
    @JoinColumn(name="studid")
    Student student;

    public Subjects(){}

    public Subjects ( String subName, int totalClasses, int classesAttended, Student student){
        //this.subId=subId;
        this.subName=subName;
        this.totalClasses=totalClasses;
        this.classesAttended=classesAttended;
        this.student=student;
    }

    public Integer getSubId() {
        return subId;
    }

    public void setSubId(Integer subId) {
        this.subId = subId;
    }

    public String getSubName() {
        return subName;
    }

    public void setSubName(String subName) {
        this.subName = subName;
    }

    public int getTotalClasses() {
        return totalClasses;
    }

    public void setTotalClasses(int totalClasses) {
        this.totalClasses = totalClasses;
    }

    public int getClassesAttended() {
        return classesAttended;
    }

    public void setClassesAttended(int classesAttended) {
        this.classesAttended = classesAttended;
    }

    @Override
    public String toString() {
        return "Subjects{" +
                "subId=" + subId +
                ", subName='" + subName + '\'' +
                ", totalClasses=" + totalClasses +
                ", classesAttended=" + classesAttended +
                '}';
    }

    public void setStudent(Student student) {
        this.student=student;
    }

    public Student getStudent(){
        return student;
    }
}

this is my subject entity class, is it possible to use subName as index instead?