You don't need to do special things in the form. You just build the comment correctly in the show action:
class ArticlesController < ActionController::Base
....
def show
@article = Article.find(params[:id])
@new_comment = @article.comments.build
end
....
end
and then make a form for it in the article view:
<% form_for @new_comment do |f| %>
<%= f.text_area :text %>
<%= f.submit "Post Comment" %>
<% end %>
by default, this comment will go to the create action of CommentsController, which you will then probably want to put redirect :back into so you're routed back to the Article page.