declare
    v_max_child_sortkey             forums_forums.max_child_sortkey%TYPE;
    v_parent_sortkey                forums_messages.tree_sortkey%TYPE;
begin

    if new.parent_id is null
    then

        select '', max_child_sortkey
        into v_parent_sortkey, v_max_child_sortkey
        from forums_forums
        where forum_id = new.forum_id
        for update;

        v_max_child_sortkey := tree_increment_key(v_max_child_sortkey);

        update forums_forums
        set max_child_sortkey = v_max_child_sortkey
        where forum_id = new.forum_id;

    else

        select coalesce(tree_sortkey, ''), max_child_sortkey
        into v_parent_sortkey, v_max_child_sortkey
        from forums_messages
        where message_id = new.parent_id
        for update;

        v_max_child_sortkey := tree_increment_key(v_max_child_sortkey);

        update forums_messages
        set max_child_sortkey = v_max_child_sortkey
        where message_id = new.parent_id;

    end if;

    new.tree_sortkey := v_parent_sortkey || v_max_child_sortkey;

    return new;
end;