title: Rails has_and_belongs_to_many always inserts into database
body: <p>Here is my problem:</p>
  
  <pre><code>class Facility &lt; ActiveRecord::Base
  ...
  has_and_belongs_to_many :languages, :autosave =&gt; false, :join_table =&gt; 'facilities_languages'
  ... 
  end
  </code></pre>
  
  <p>When I do something like this:   </p>
  
  <pre><code>facility = Facility.find(1)
  language = Language.find(1)
  facility.languages &lt;&lt; language
  </code></pre>
  
  <p>Rails always do the SQL request:</p>
  
  <pre><code>"INSERT INTO `facilities_languages` (`language_id`,`facility_id`) VALUES (1, 1)"
  </code></pre>
  
  <p>Is there a way to avoid database requests unless I call  'facility.save' ?</p>
  
  <p>Apparently, :autosave option here does something else.</p>