belongsTo(Subject::class, 'subject_id'); } public function category() { return $this->belongsTo(Category::class, 'category_id'); } public function skill() { return $this->belongsTo(Skill::class, 'skill_id'); } public function questionGroups() { return $this->hasMany(QuestionGroup::class, 'exercise_id'); } public function questions() { return $this->hasMany(Question::class, 'exercise_id'); } }