public bool DelTeaching(int lecturerId, int courseId)
        {
            Teaching teaching = _ctx.Teachings.Find(lecturerId, courseId);

            if (teaching != null)
            {
                _ctx.Teachings.Remove(teaching);
            }
            return(save()
                   );
        }
        public void AddTeaching(int lecturerId, int courseId)
        {
            Lecturer lecturer = _ctx.Lecturers.Find(lecturerId);
            Course   course   = _ctx.Courses.Find(courseId);
            Teaching teaching = new Teaching
            {
                LecturerId = lecturerId,
                CourseId   = courseId,
                Lecturer   = lecturer,
                Course     = course
            };

            _ctx.Teachings.Add(teaching);
            save();
        }