Best way to handle user's topic views in a forum

Hello,

I'm french... hope you will understand my English ^^

So I'm building a forum for my website from scratch. None of the
already built forum like Beast or RForum suits my needs, and it'll
take as much time to integrate them as to create a new one.

I would like your opinion on how to handle the user's topic views.
The best behavior would be that every unread topic stays in this state
as long as the user doesn't read it, and everytime a new post is
created the topic is marked unread for that specific user. But I think
it'll take to much resources to do that.
So I'm thinking to do it the "punbb" way (not really sure but I
remember it that way) : when a user goes back to the forum, every
topic updated since its last visit is marked unread, until the user
reads it. The only thing I don't like with this approach is that I
need to update an "last_visit_at" attribute on my users every time
they request a page on the forum.
So how would you do it ?

My structure looks like this :

   1. class Forum < ActiveRecord::Base
   2. has_many :forum_topics
   3. has_many :forum_posts
   4. end
   5.
   6. class ForumTopic < ActiveRecord::Base
   7. belongs_to :user
   8. belongs_to :forum
   9. has_many :forum_posts
  10. end
  11.
  12. class ForumPost < ActiveRecord::Base
  13. belongs_to :user
  14. belongs_to :forum
  15. belongs_to :forum_topic
  16. end

Thanks

Thibaut