CourseWork/app/Services/SubjectTeacherService.php

<?php

namespace App\Services;

use App\Models\Subject;
use App\Models\Teacher;
use Illuminate\Database\Eloquent\Collection;
use Illuminate\Database\Eloquent\Model;

class SubjectTeacherService implements ServiceInterface
{
    public function getAll(?Teacher $teacher = null): Collection
    {
        return $teacher->subjects;
    }

    public function getAllSubjects(): Collection
    {
        return Subject::all();
    }

    public function getGrades(Teacher $teacher, Subject $subject): Collection
    {
        return $teacher
            ->grades()
            ->join('grade_subject', 'grades.id', '=', 'grade_subject.grade_id')
            ->where('subject_id', $subject->id)->get();
    }

    public function create(array $data, ?Model $model = null): Teacher
    {
        $model->subjects()->syncWithoutDetaching($data['subject_id']);

        return $model;
    }

    public function update(Model $model, array $data, ?Model $subject = null): Teacher
    {
        $model->subjects()->detach($subject->id);
        $model->subjects()->attach($data['subject_id']);

        return $model;
    }

    public function delete(Model $model, ?Model $subject = null): Teacher
    {
        $model->subjects()->detach($subject);

        return $model;
    }
}