public HttpResponseMessage LoginUser(UserModel model)
        {
            return this.ExecuteOperationAndHandleExceptions(() =>
            {
                this.ValidateUser(model);

                var data = new UserRepository(
                ConfigurationManager.AppSettings["MongoConnectionString"],
                ConfigurationManager.AppSettings["Database"]);

                var dbUser = new DbUserModel()
                {
                    Username = model.Username,
                    AuthCode = model.AuthCode
                };

                var user = data.GetLoggedUser(dbUser);

                if (user == null)
                {
                    throw new FormatException("Invalid username or password");
                }

                string token = null;

                if (user.AccessToken == null)
                {
                    token = data.SetAccessToken(user, this.GenerateAccessToken(user.Id));
                }
                else
                {
                    token = user.AccessToken;
                }

                var responseModel = new LoginResponseModel()
                {
                    Id = user.Id,
                    Username = user.Username,
                    AccessToken = token
                };

                var response = this.Request.CreateResponse(HttpStatusCode.OK, responseModel);
                return response;
            });
        }