Rails nested form with has_many :through, how to edit attributes of join model?

Question

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 :argument
  belongs_to :question
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?

Answer 1

14 days later - Figured out the answer. Hopefully my next question will get some response. The trick was:

@topic.linkers.build.build_article

That builds the linkers, then builds the article for each linker. So, in the models:
topic.rb needs "accepts_nested_attributes for :linkers"
linker.rb needs "accepts_nested_attributes for :articles"

Then in the form:

<% form_for(@topic) do |topic_form| %>
  ...fields...
  <% topic_form.fields_for :linkers do |linker_form| %>
    ...linker fields...
    <% linker_form.fields_for :article do |article_form| %>
      ...article fields...

Answer 2

I was just looking to do the same sort of thing! I've got two models:

Songs and Performances and they are joined by PlayedSongs.

But I've got additional fields besides song_id and performance_id on the PlayedSongs. One of which is "play_order" because the songs need to be displayed in the order they were played live (I'm working on a site to catalog live performances) and I couldn't figure out how to do it automagically...somehow having Rails auto-fill the played_order field with a loop starting at 1 and adding 1 for each additional song added to the performance.

This is a big help (I hope) toward making it a bit easier...right now I've got a separate action setup with a separate view that users must go to and manually enter the play_order after adding the songs to the show...this might allow me to bring that up one level and have it right on the page when they're first adding the songs.

I know this isn't helping you at all and for that I apologize, I just wanted to thank you for posting this after you figured it out.

I have a question though. Did this:

@topic.linkers.build.build_article

Replace

@topic.articles.build

In your New action then? Or are both lines still needed? And are you calling both

accepts_nested_attributes_for :article
accepts_nested_attributes_for :linker

on the topic model? Or just the line for :linker? If just :linker, is it because Rails is smart enough to know that Articles are tied to Linkers with the HMT relationship and therefore grants you access to that model as well?