Associating posts with page in menu

Hi

I’m trying to associate posts with a parent page to maintain menu context when viewing said post.

At the moment, I have a page called portfolio. It appears in the menu. I have a collection of posts, in the category portfolio. The urls for the posts is /%category%/%postname%/, which gives a hierarchy to the urls – the posts appear as child items of the portfolio page.

What I need is the menu to maintain is state when viewing a post. Portfolio is highlighted when on the corresponding page, but I need it to remain so when viewing a post from the portfolio category. I’m not looking to display the posts as a submenu, simply maintain the highlighting of the current node.

I don’t want to use a plugin, if possible. Any suggestions would be greatly appreciated.

cheers
Nathan