Hi John,
I highly suggest you take a peek as the associations that Rails provides. A good place to start is the Rails Guides.
and also the guide on routes (particularly the part on nested resources) http://guides.rubyonrails.org/routing_outside_in.html These will make life much easier for you if you follow them.
Sorry for not really answering your question... 
Again, I would go this route though.
I think you have to have a look at Rails basics. A basic forum would be something like:
forums has_many categories categories has_many topics topics has_many replies
then you have has_many routes like /forums/1/categories/1/topic and in your routes.rb you would do
map.resources :forums do |forum|   forum.resources :categories do |cat|     cat.resources :topics   end end
You could add :has_many => :topics but if you need members and/or collections later its better this way.