title: has_many through checkboxes
body: <p>Given the following models</p>
  
  <pre><code>class Feed &lt; ActiveRecord::Base  
    has_many :alerts , :dependent =&gt; :destroy
    has_many :users , :through =&gt; :alerts
  end
  
  class Alert &lt; ActiveRecord::Base  
    belongs_to :user
    belongs_to :feed  
    has_and_belongs_to_many :alerttypes
  
  end 
  
  class Alerttype &lt; ActiveRecord::Base
    has_and_belongs_to_many :alerts
  end
  </code></pre>
  
  <p>I can create a form to list the Alerttypes when adding a new Feed and this submits correctly. I am however unable to update these Alerttypes successfully. I cannot seem to find the correct method for defining the checkbox naming</p>
  
  <pre><code>&lt;%= check_box_tag "UNKNOWN", type.id , @alerts[0].alerttypes.include?(type)%&gt;
  </code></pre>
  
  <p>The checking of the Alerttypes being 'checked' works but any changes in those Alerttypes when editing does not update and they stay as when first created.</p>