Saving join attributes through a has_many :through with :conditions

Question

I have an Artist model that looks like this:

# app/models/artist.rb
class Artist < ActiveRecord::Base
  # Relationships
  has_many  :releases
  has_many  :songs, :through => :releases
  has_many  :featured_songs,  :through => :releases,
                              :class_name => "Song",
                              :source => :song,
                              :conditions => { 'releases.featured', true }                         

end

Retrieving the featured_songs works perfectly. The issue here is I'm unable to add a new featured_song to an artist because for some reason the 'featured' attribute is set to 'nil'.

This is what I'm attempting:

ruby-1.9.2-p180 :004 > a = Artist.first
ruby-1.9.2-p180 :005 > a.featured_songs.create(:title => "Title", :user => User.first)

The actual result of that is:

ruby-1.9.2-p180 :004 > a = Artist.first
ruby-1.9.2-p180 :005 > a.featured_songs.create(:title => "Title", :user => User.first)
  User Load (0.9ms)  SELECT `users`.* FROM `users` LIMIT 1
  SQL (1.0ms)  BEGIN
  SQL (5.5ms)  INSERT INTO `songs` (`created_at`, `title`, `updated_at`, `user_id`) VALUES (?, ?, ?, ?)  [["created_at", Thu, 11 Aug 2011 18:30:34 UTC +00:00], ["title", "Title"], ["updated_at", Thu, 11 Aug 2011 18:30:34 UTC +00:00], ["user_id", 1]]
  SQL (1.2ms)  INSERT INTO `releases` (`album_id`, `artist_id`, `created_at`, `featured`, `song_id`, `updated_at`) VALUES (?, ?, ?, ?, ?, ?)  [["album_id", nil], ["artist_id", 1], ["created_at", Thu, 11 Aug 2011 18:30:34 UTC +00:00], ["featured", nil], ["song_id", 6], ["updated_at", Thu, 11 Aug 2011 18:30:34 UTC +00:00]]
   (0.1ms)  COMMIT

Notice the: ["featured", nil]

Any idea what i'm doing wrong? How can i properly set attributes on my join without accessing it directly?

thank you!

EDIT: To make my issue more clear:

• From an instance of artist i am unable to create new featured songs through the featured_songs relationship

• Saves appear to be setting all of the song attributes EXCEPT for (the most important one) featured

• The featured attribute is being set to nil for some reason and this is the real issue here.

Answer 1

I found the working solution after reading this:

https://github.com/rails/rails/issues/5178#issuecomment-4181551

The trick is to create a new first-order has_many association that contains the condition, and then run the has_many :through on that.

So, in your case, it would be:

class Artist < ActiveRecord::Base
  # Relationships
  has_many  :releases
  has_many  :songs, :through => :releases

  has_many  :featured_releases, :class_name => "Release", :conditions => { :featured => true }
  has_many  :featured_songs, :through => :featured_releases, :source => :song
end

Answer 2

The key to allow save through association is to provide attr_accessible to fields on association model.

In this case, add line on Release model:

attr_accessible :song_id, :feature_id

Answer 3

EDIT: Updated :releases_attributes to reflect has_many relationship.

jaydel's answer is along the right lines. This is a problem solved by accepts_nested_attributes_for. has_many only describes a mechanism to query records, not to update or create them.

The reason the attributes you listed are being updated is because they are part of the song object. The field you're trying to update is the featured field, which is part of your releases join table.

accepts_nested_attributes_for is the tool to solve the problem of updating nested relationships. If you want to add the featured attribute during creation of a song object (as you do in your example), then you need to add that as part of the nested attributes for the Song class.

class Song < ActiveRecord::Base
  has_many :releases
  has_many :artists, :through => :releases
  accepts_nested_attributes_for :releases
end

Your create would then look something like this:

a.songs.create(:title => 'Title', :user => User.first, :releases_attributes => [{ :id => 123, :featured => true}])

Note the addition of _attributes. This is exactly what Rails would do in the views if you were using nested forms. See the ActiveRecord source file nested_attributes.rb for proof.

If you really wanted a cleaner way to accomplish this, you could add a Song#create_featured method that would merge the :releases_attributes hash into the options to #create.

Answer 4

You can use accepts_nested_attributes_for in the model. I would start by watching the Railscast on the topic and then peruse the documentation itself for the method

EDIT:

From the comments below it's clear I don't completely understand what you're asking for. I thought you wanted to know how to create and manipulate a joined object from a model. When I needed to do this recently I had these two models:

class User < ActiveRecord::Base
  belongs_to :address

  accepts_nested_attributes_for :address
end

class Address < ActiveRecord::Base
  has_one :user
end

Then I can do this to create a new address on a user

u = User.new
u.address.build({:city => "bozoville", :zip => "22222", ...})
u.save!

and my address is on my user and persisted. Is this what you were looking for? I will say that I'm not entirely how effectively this translates to a many to many association, but I was assuming it would be the same thing. Perhaps because the one to one relies on a single column for the foreign key that works, but when it involves a more complex assocation with an actual join table it doesn't work? At the very least you might investigate the possibility further...