Trouble on saving nested models

I am using Ruby on Rails 3.0.7 and I am trying to save an "has_many :through => checkboxes" class object (I have read the Quick Tip: has_many :through => checkboxes blog post) but I have some troubles on doing that. I would like to create some associations between a new article and article categories "owned" by a user (in the below code the @current_user) taking advantage of the "magical way" of doing that with Ruby on Rails association models.

In my models I have:

class Article < ActiveRecord::Base
  has_many :category_relationships,
    :class_name  => 'Categories::ArticleRelationship',
    :foreign_key => 'article_id',
    :autosave    => true,
    :dependent   => :destroy
  has_many :article_categories,
    :through     => :category_relationships,
    :source      => :article_category,
    :uniq        => true,
    :dependent   => :destroy

  attr_accessible :category_relationship_ids
end

class Categories::ArticleRelationship < ActiveRecord::Base
  belongs_to :article,
    :class_name  => 'Article',
    :foreign_key => 'article_id'

  belongs_to :article_category,
    :class_name  => 'Articles::Category',
    :foreign_key => 'category_id'
end

In my view file I have:

...

<% @current_user.article_categories.each do |article_category| %>
  <div>
    <%= check_box_tag :category_relationship_ids, article_category.id, false, :name => 'article[category_relationship_ids][]' %>
    <%= label_tag :article_category, article_category.name %>
  </div>
<% end %>

...

that outputs the following HTML code:

<div>
  <input type="checkbox" value="5" name="article[category_relationship_ids][]" id="category_relationship_ids">
  <label for="category">comunication</label>
</div>
<div>
  <input type="checkbox" value="6" name="article[category_relationship_ids][]" id="category_relationship_ids">
  <label for="category">internal</label>
</div>

When I check\select the above two check boxes, (then) I submit the form and I inspect the log file (outputting @article and @article_relationships data respectively related to Article and Categories::ArticleRelationship object instances), I get the following:

param[:article]        => {"name"=>"Sample title", "category_relationship_ids"=>["5", "6"], ...}
@article               => #<Article id: nil, title: "Sample title", ...>
@article_relationships => [#<Categories::ArticleRelationship id: 5, category_id: 6, article_id: 3, ...>, [#<Categories::ArticleRelationship id: 6, category_id: 4开发者_开发问答, article_id: 5, ...>

It seems that Ruby on Rails set Categories::ArticleRelationship id values to check box values (in the above HTML code: 5 and 6) instead of set those to nil (those should be nil because Categories::ArticleRelationship are not created yet in the database). Furthermore, I don't know where it takes from the category_id and article_id values (article_id values should be nil because @article instances are not created yet in the database).

What is the problem? How can I solve that?