public Operation <StaffModel> UpdateStaff(StaffModel model)
        {
            return(System.Operation.Create(() =>
            {
                //model.Validate();
                var isExist = _context.Staffs.Find(model.StaffId);
                if (isExist == null)
                {
                    throw new Exception("staff does not exist");
                }

                var entity = model.Edit(isExist, model);
                _context.Entry(entity);
                _context.SaveChanges();
                return model;
            }));
        }