Structuring a Rails app for Russian Doll Caching with a has_many relationship

After studying DHH's and other blog articles about key-based cache expiration and Russian Doll Caching, I am still unsure how to handle one relation type. To be specific, a has_many relationship.

I will share the results of my research on a sample app. It is a little bit of story telling, so hang on. Let's say we have the following ActiveRecord models. All we care about is a proper change of the model's cache_key, right?

class Article < ActiveRecord::Base
  attr_accessible :author_id, :body, :title
  has_many :comments
  belongs_to :author
end

class Comment < ActiveRecord::Base
  attr_accessible :article_id, :author_id, :body
  belongs_to :author
  belongs_to :article, touch: true
end

class Author < ActiveRecord::Base
 attr_accessible :name
  has_many :articles
  has_many :comments
end

We already have one article, with one comment. Both by a different author. The goal is to have a change in the cache_key for the article in the following cases:

1. Article's body or title changes
2. Its comment's body changes
3. Article's author's name changes
4. Article's comment's author's name changes

So by default, we are good for case 1 and 2.

1.9.3-p194 :034 > article.cache_key
 => "articles/1-20130412185804"
1.9.3-p194 :035 > article.comments.first.update_attribute('body', 'First Post!')
1.9.3-p194 :038 > article.cache_key
 => "articles/1-20130412185913"

But not for case 3.

1.9.3-p194 :040 > article.author.update_attribute('name', 'Adam A.')
1.9.3-p194 :041 > article.cache_key
 => "articles/1-20130412185913"

Let's define a composite cache_key method for Article.

class Article < ActiveRecord::Base
  attr_accessible :author_id, :body, :title
  has_many :comments
  belongs_to :author

  def cache_key
    [supe