public UserDTO Map(User user)
 {
     return new UserDTO
         {
             UserId = user.UserId,
             UserName = user.UserName,
             CreatedAt = user.CreatedAt,
             ModifiedAt = user.ModifiedAt
         };
 }
        public UserDTO CreateUser(BlogContext context, string userName, string password)
        {
            var existingUser = context.Users.SingleOrDefault(u => u.UserName == userName);
            if (existingUser != null)
            {
                throw new UserNameAlreadyRegisteredException();
            }

            var user = new User
                {
                    UserName = userName,
                    Password = password,
                    CreatedAt = DateTime.UtcNow,
                    ModifiedAt = null
                };
            user = context.Users.Add(user);
            context.SaveChanges();

            return _userToUserDtoMapper.Map(user);
        }