What would be a best association for this?

I am making a simple BBS (Bulletin Board System) with ruby on rails3.

3 main modesl are Members/Categories/Articles.

Article 'belongs_to' Members
Article 'belongs_to' Categories,

Article then have member_id/category_id columns in db table.

Member 'has_many' articles.
Category 'has_many' articles.

When a specific user tries to write an article, I tried it by

def new
  @article = current_member.articles.new
end
and that automatically filled in a member_id section when an article
is created without any form input or anything.

Now, what should I do if I want to automatically fill a category_id
column of an article?? I believe every data related jobs should be
done within model. However, I am passing in :category value through
url

For example,

localhost:3000/articles/qna/new

would mean the article should have an category_id of 2 (assuming
category with id=2 has name=qna, also, I did routing jobs that I can
successfully get 'qna' from params[:category]).

Should I use

def create
   current_member.articles.build(:category =>
get_category_id_from_name(params[:category]))
end
?

But is it okay? because I believe since models cannot access params
variable, controller has to do the above job, and thats not 'rails
way' I do not want to use nested form, because I do not want user to
choose an category when they are writing. Its like, if there is a QnA
board, and if user clicked 'write' button, that means user is writing
in a QnA board.