Ruby on Rails

I Copied this from StackOverflow since I did not get any response there. There is an update at the end. 

I am not sure why my checkboxes for a has_many through are not saving. This is a rather complex form (An online application for a language summer program) so in addition to regular applicant information, I am collecting attributes of a LanguageBackgroundAndInterest model using a fields_for. Part of these attributes are the books that the applicant used to learn the language. The code looks like this:

    <%= f.fields_for :language_background_and_interest do |builder| %>        <div class="field">         <%= hidden_field_tag "language_background_and_interest[book_ids][]" %>        <% Book.all.each do |book| %>          <br><%= check_box_tag "language_background_and_interest[book_ids][]", book.id %>          <%= book.name.humanize %>        <% end %>        </div>      <% end %>

Both the language_background_and_interest and books are joined together using a has_many_through like so:

class LanguageBackgroundAndInterest < ActiveRecord::Base    attr_accessible :book_ids      has_many :language_background_books    has_many :books, through: :language_background_books  end    class Book < ActiveRecord::Base    attr_accessible :name, :publisher      has_many :language_background_books    has_many :language_background_and_interests, through: :language_background_books  end    # Join Table  class LanguageBackgroundBook < ActiveRecord::Base    attr_accessible :language_background_and_interest_id, :book_id    belongs_to :language_background_and_interest    belongs_to :book  end

I am not sure why the books from the checkboxes don't get assigned to the appropriate background model. Any ideas?

PS: Granted, this design is rather ambiguous (Why not make books belong to an applicant?), but I currently want to put up a prototype and I am also constrained by a dubious requirement. So I need this working.

Update: 

I appended the word "attributes" bellow to the language_background model in the hope that the books_ids array gets inserted properly into the params hash with the other attributes. Inspecting the development.log, I can see the key language_background_and_interest_attributes twice: One for all the other data which get successfully saved, and one for the book_ids array which apparently gets ignored. Any ideas?

    <%= f.fields_for :language_background_and_interest do |builder| %>        <div class="field">         <%= hidden_field_tag "language_background_and_interest_attributes[book_ids][]" %>        <% Book.all.each do |book| %>          <br><%= check_box_tag "language_background_and_interest_attributes[book_ids][]", book.id %>          <%= book.name.humanize %>        <% end %>        </div>      <% end %>

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