Ruby-on-rails – Rails nested form with has_many :through, how to edit attributes of join model

How do you edit the attributes of a join model when using accepts_nested_attributes_for?

I have 3 models: Topics and Articles joined by Linkers

class Topic < ActiveRecord::Base
  has_many :linkers
  has_many :articles, :through => :linkers, :foreign_key => :article_id
  accepts_nested_attributes_for :articles
end
class Article < ActiveRecord::Base
  has_many :linkers
  has_many :topics, :through => :linkers, :foreign_key => :topic_id
end
class Linker < ActiveRecord::Base
  #this is the join model, has extra attributes like "relevance"
  belongs_to :topic
  belongs_to :article
end

So when I build the article in the "new" action of the topics controller…

@topic.articles.build

…and make the nested form in topics/new.html.erb…

<% form_for(@topic) do |topic_form| %>
  ...fields...
  <% topic_form.fields_for :articles do |article_form| %>
    ...fields...

…Rails automatically creates the linker, which is great.
Now for my question: My Linker model also has attributes that I want to be able to change via the "new topic" form. But the linker that Rails automatically creates has nil values for all its attributes except topic_id and article_id. How can I put fields for those other linker attributes into the "new topic" form so they don't come out nil?