public async Task<IHttpActionResult> PutCours(int id, Cours cours)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            if (id != cours.Id)
            {
                return BadRequest();
            }
            var dbCours = db.Courses.FirstOrDefault(o => o.Id == cours.Id);

            var once = true;
            foreach(var student in cours.Students)
            {

                var s = db.Students.FirstOrDefault(o => o.Id == student.Id);
                if (once) { 
                dbCours.Students.Clear();
                    once = false;
                }
                dbCours.Students.Add(s);
                
            }
            
            dbCours.Name = cours.Name;
            dbCours.Level = cours.Level;
            dbCours.Code = cours.Code;
            //dbCours.Students = cours.Students;

            db.Entry(dbCours).State = EntityState.Modified;

            try
            {
                await db.SaveChangesAsync();
            }
            catch (DbUpdateConcurrencyException)
            {
                if (!CoursExists(id))
                {
                    return NotFound();
                }
                else
                {
                    throw;
                }
            }

            return StatusCode(HttpStatusCode.NoContent);
        }
        public async Task<IHttpActionResult> PostCours(Cours cours)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            db.Courses.Add(cours);
            await db.SaveChangesAsync();

            return CreatedAtRoute("DefaultApi", new { id = cours.Id }, cours);
        }