sub treeify {
    my ($sth) = @_;

    my %root;
    my %children;

    while (my $row = $sth->fetch()) {
        my ($id, $parent_id) = @$row;

        my $parent = ( defined($parent_id)
            ? $children{$parent_id} ||= {}
            : \%root
        );

        $parent->{$id} = $children{$id} ||= {};
    }

    return \%root;
}