Get the effective user, especially during impersonation.
        :param url: SQL Alchemy URL object
        :param user_name: Default username
        :return: The effective username

def get_effective_user(self, url, user_name=None):
        """
        Get the effective user, especially during impersonation.
        :param url: SQL Alchemy URL object
        :param user_name: Default username
        :return: The effective username
        """
        effective_username = None
        if self.impersonate_user:
            effective_username = url.username
            if user_name:
                effective_username = user_name
            elif (
                hasattr(g, 'user') and hasattr(g.user, 'username') and
                g.user.username is not None
            ):
                effective_username = g.user.username
        return effective_username