//add manytomany to db

        public static CourseTeacher BindTeacherToCourse(int teacherId, int courseId)
        {
            CoursesEntities db = new CoursesEntities();

            if (db.CourseTeachers.Where(p => p.TeacherId == teacherId && p.CourseId == courseId).Count() != 0)
            {
                return(null);
            }
            CourseTeacher newCourseTeacher = new CourseTeacher();

            newCourseTeacher.CourseId  = courseId;
            newCourseTeacher.TeacherId = teacherId;
            db.CourseTeachers.Add(newCourseTeacher);
            try
            {
                db.SaveChanges();
                db.Dispose();
            }
            catch (System.Data.Entity.Validation.DbEntityValidationException)
            {
                db.Dispose();
                return(null);
            }
            return(newCourseTeacher);
        }
        //delete manytomany from db

        public static bool DelBindTeacherToCourse(int id)
        {
            CoursesEntities db            = new CoursesEntities();
            CourseTeacher   courseTeacher = db.CourseTeachers.Find(id);

            if (courseTeacher != null)
            {
                db.CourseTeachers.Remove(courseTeacher);
                try
                {
                    db.SaveChanges();
                    db.Dispose();
                    return(true);
                }
                catch (Exception)
                {
                    db.Dispose();
                    return(false);
                }
            }
            return(false);
        }