title: Showing check box values in edit page
body: <p>I'm using the following collection_select in the new and edit pages to let users select contacts and associate them to a group:</p>
  
  <pre><code>&lt;%= f.label :contacts %&gt;
      &lt;div&gt;&lt;span class="ul"&gt;
        &lt;% current_user.contacts.all.each do |contact| %&gt;
        &lt;%= check_box_tag "contacts[]", contact.id %&gt;
        &lt;%= f.label contact.name %&gt;
        &lt;% end %&gt;&lt;/div&gt;
      &lt;/span&gt;
  </code></pre>
  
  <p>I would like to show the already selected contacts checked in the edit page of the group. Is there another parameter that I can use along with the check_box_tag?</p>