title: Timestamps on HABTM relationships with ActiveRecord
body: <p>I have the following relationships setup:</p>
  
  <pre><code>class Article &lt; ActiveRecord::Base
    has_and_belongs_to_many :authors
  end
  
  class Author &lt; ActiveRecord::Base
    has_and_belongs_to_many :articles
  end
  </code></pre>
  
  <p>I noticed that, although the join table <code>articles_authors</code> has timestamps, they do not populate when creating a new relationship. For example:</p>
  
  <pre><code>Author.first.articles &lt;&lt; Article.first
  </code></pre>
  
  <p>It's important that I keep track of when an author is associated with an article.
  Is there a way that I can do this?</p>