Mirrors the user's LDAP groups in the Django database and updates the
        user's membership.

def _mirror_groups(self):
        """
        Mirrors the user's LDAP groups in the Django database and updates the
        user's membership.
        """
        target_group_names = frozenset(self._get_groups().get_group_names())
        current_group_names = frozenset(self._user.groups.values_list('name', flat=True).iterator())

        if target_group_names != current_group_names:
            existing_groups = list(Group.objects.filter(name__in=target_group_names).iterator())
            existing_group_names = frozenset(group.name for group in existing_groups)

            new_groups = [Group.objects.get_or_create(name=name)[0] for name
                          in target_group_names if name not in existing_group_names]

            self._user.groups = existing_groups + new_groups