title: Override default scope in active_admin form.has_many
body: <p>Given a model Post that has_many attachments, and an attachment has a hidden flag. Throughout the app I want to easily say post.attachments and only get the visible ones, so i setup a default scope in the Attachment model (using squeel):</p>
  
  <pre><code>default_scope -&gt; { where { (hidden != true) | (hidden == nil) } } 
  </code></pre>
  
  <p>But the admin page needs to be able to see all attachments for a post, not just the visible ones (so you can toggle the hidden checkbox). The default way of doing this (in admin/posts.rb) uses the default_scope and only lets me edit the visible ones:</p>
  
  <pre><code>f.has_many :attachments do |a|
    ...
  end
  </code></pre>
  
  <p>I know I could just not use default_scope and instead name it :visible, and then everywhere (except the admin page) say post.attachments.visible but I prefer not having the do that.</p>
  
  <p>How can I unscope the children attachments on the admin page?</p>