public HttpResponseMessage PostRegisterUser(UserModel model)
        {
            var responseMsg = this.PerformOperationAndHandleExceptions(() =>
            {
                var context = new MyForumContext();
                using (context)
                {
                    this.ValidateUsername(model.Username);
                    this.ValidateNickname(model.Nickname);
                    this.ValidateAuthCode(model.AuthCode);

                    var usernameLower = model.Username.ToLower();
                    var nicknameLower = model.Nickname.ToLower();

                    var user = context.Users
                        .FirstOrDefault(usr => usr.Username == usernameLower ||
                            usr.Nickname == nicknameLower);

                    if (user != null)
                    {
                        throw new InvalidOperationException("User exists");
                    }

                    user = new User()
                    {
                        Username = usernameLower,
                        Nickname = model.Nickname,
                        AuthCode = model.AuthCode,
                    };

                    context.Users.Add(user);
                    context.SaveChanges();

                    user.SessionKey = this.GenerateSessionKey(user.Id);
                    context.SaveChanges();

                    var loggedModel = new LoggedUserModel()
                    {
                        Nickname = user.Nickname,
                        SessionKey = user.SessionKey
                    };

                    var response = this.Request.CreateResponse(
                        HttpStatusCode.Created, loggedModel);

                    return response;
                }
            });

            return responseMsg;
        }
        public HttpResponseMessage PostLoginUser(UserModel model)
        {
            var responseMsg = this.PerformOperationAndHandleExceptions(() =>
            {
                var context = new MyForumContext();
                using (context)
                {
                    this.ValidateUsername(model.Username);
                    this.ValidateAuthCode(model.AuthCode);

                    var usernameLower = model.Username.ToLower();
                    var nicknameLower = model.Nickname.ToLower();

                    var user = context.Users
                        .FirstOrDefault(
                        usr => usr.Username == usernameLower &&
                            usr.AuthCode == model.AuthCode);

                    if (user == null)
                    {
                        throw new InvalidOperationException("Invalid name or password");
                    }

                    if (string.IsNullOrWhiteSpace(user.SessionKey))
                    {
                        user.SessionKey = this.GenerateSessionKey(user.Id);
                        context.SaveChanges();
                    }

                    var loggedModel = new LoggedUserModel()
                    {
                        Nickname = user.Nickname,
                        SessionKey = user.SessionKey
                    };

                    var response = this.Request.CreateResponse(
                        HttpStatusCode.OK, loggedModel);

                    return response;
                }
            });

            return responseMsg;
        }