sub insert_node {
    my ($class, $rs, $id, $data) = @_;
    my $var = $class . "::_nested_set";
    no strict 'refs';
    my ($left_id, $right_id, $parent_id) = (1, 2, 0);
    $data->{title} = 'root' unless defined $data->{title};
    if ($id) {
        my $cat = $rs->find($id) or return;
        my $upper = $rs->search({
            left_id => { '<=' => $cat->left_id },
            right_id => { '>=' => $cat->right_id },
        });
        $upper->update({
            right_id => \'right_id + 2',
        });
        my $right = $rs->search({
            left_id => { '>' => $cat->left_id },
            right_id => { '>' => $cat->right_id },
        });
        $right->update({
            left_id => \'left_id + 2',
            right_id => \'right_id + 2',
        });
        ($left_id, $right_id, $parent_id) = ($cat->right_id, $cat->right_id + 1, $cat->id);
    }
    my $node = $rs->create({
        %$data,
        left_id => $left_id,
        right_id => $right_id,
        parent_id => $parent_id,
    });
    return $node;
}

($left_id, $right_id, $parent_id) = ($cat->right_id, $cat->right_id + 1, $cat->id);