public Student Assign(int stuId, int couId)
        {
            Student student = Find(stuId);

            foreach (var course in student.PersonCourses)
            {
                if (course.Course.Id == couId)
                {
                    return(student);
                }
            }

            PersonCourse personCourse = new PersonCourse
            {
                CourseId  = couId,
                StudentId = stuId
            };

            return(_studentRepository.Assign(personCourse));
        }