public string Create(string dm, DanhMuc model)
 {
     if (ModelState.IsValid)
     {
         var danhmuc = db.makeDanhMuc(dm, model);
         TryValidateModel(danhmuc);
         if (ModelState.IsValid)
         {
             if (model.stt.HasValue)
             {
                 db.updateSTT(dm, model.stt.Value);
             }
             db.Entry(danhmuc).State = EntityState.Added;
             db.SaveChanges();
             // for unit testing
             model.id = (int)danhmuc.GetType().GetProperty("id").GetValue(danhmuc, null);
             return("OK");
         }
     }
     foreach (var state in ModelState.Values)
     {
         if (state.Errors.Count > 0)
         {
             return(state.Errors[0].ErrorMessage);
         }
     }
     return("ModelState Invalid!");
 }