private void Handle(FeedDataSourceSubscribedEvent evt)
        {
            if (UserId != evt.UserId)
            {
                throw new DomainException(Global.UserNotAuthorizedToSubscribe);
            }

            if (DataSources.Any(d => d.DatasourceId == evt.DataSourceId))
            {
                throw new DomainException(string.Format(Global.DatasourceAlreadyExists, evt.DataSourceId));
            }

            DataSources.Add(FeedDatasource.Create(evt.DataSourceId));
            UpdateDateTime = evt.CreateDateTime;
            Version        = evt.Version;
        }