Export an Excel sheet in Laravel ?

I am trying to Export an Excel sheet but I have a problem with this line

$row[]=$student['student']['user']['en_name'];

This is the relation between user and student

public function user()
    {
        return $this->belongsTo(User::class,'user_id', 'id');
    }

public function getStudentExposrtGrades($course)
    {
         //to get offering grade items heading grades
         $courseModel=Course::find($course);
         $columns=$courseModel->gradeItems()->pluck('grade_items.id')->toArray();
         $query=[];
         $courseStudentsGrades=CourseStudent::select('id','student_id','course_id')
         ->with('student:id,student_code','gradeItemCourses:id','student.user:id,en_name')
         ->where('course_id',$course)
         ->get()->toArray();
         foreach($courseStudentsGrades as $student){
             $row=[];
             $row[]=$student['student']['student_code'];
             $row[]=$student['student']['user']['en_name'];
             $totalGrades=0;
                $row[]=$student_grade;
             }
         }
         return $query;
        //dd($query);
   
    }