cases in rules problem

I am trying to make a sql based versioning system.
I am working on a Ruby on Rails project and am using a plugin called
hobo the plugin can do some nice things but over all its lame but
thats what i got  to work with.
The problem is hobo does a lot of work for you but the database most
be in a standard format to use it.
so my idea for a sql versioning  work around was this.

CREATE TABLE main(
    id serial CONSTRAINT firstkey PRIMARY KEY,
    parent_id int,
    title varchar(30),
    public boolean default false
    );

INSERT INTO main(parent_id,title,public)
VALUES
    (1,'blah',true),
    (1,'tah',false),
    (1,'blah2',false),
    (1,'blah3',false),
    (2,'tah2',false),
    (2,'tah3',true);

CREATE VIEW  vmain as
(SELECT * FROM main
        WHERE public=true
        ORDER BY id DESC)
        UNION
(SELECT *
        FROM main
        WHERE id IN (select max(id) from main group by parent_id)
        ORDER BY id DESC)

CREATE OR REPLACE RULE main_up AS ON UPDATE TO vmain
DO INSTEAD
INSERT INTO main(parent_id,title,public)
VALUES(NEW.parent_id,NEW.title,false);

the result of the view should be all rows with public as true and one
false for each new parent_id  if any that must have a higher id than
the true one.

So on the web server, someone of level writer can edit something a
superuser has created but what happens is it puts the update into the
view hits the rule and makes a dup in the main table with public set
to false so no one on the outside can see it. And basically the most
rows that show up will be the public on and the highest id private
one i don't really care about them rolling back versions.

My problem is when the admin wants to approve the private row.  I tryed
CREATE OR REPLACE RULE main_up AS ON UPDATE TO vmain
DO INSTEAD
CASE NEW.public = true and OLD.public = false
THEN
    UPDATE main set public=true where id=NEW.id
ELSE
INSERT INTO main(parent_id,title,public)
VALUES(NEW.parent_id,NEW.title,false);

But i can't seem to put CASE statements in a rule is there any why i
can do then with out having to create a function and rule that fires it?
This has to  go on alot of table.