title: Paperclip processors for new instances
body: <p>I'm struggling a lot with papperclip and custom cropping.</p>
  
  <p>I'm using lambda for styles and a custom processor like the code below.
  But when I run <code>User.new(user_params)</code>, <code>a.instance</code> is an instance of <code>User</code>, but empty, with none of the values passed by <code>user_params</code> like <code>width</code>, <code>top</code> or <code>left</code>.</p>
  
  <p>After saving, the data is there, but cropping won't work because it appears paperclip ignores what is the instance before saving.</p>
  
  <pre class="lang-rb prettyprint-override"><code>  has_attached_file :image,
      styles: lambda {|a| {
        croped: {processors: [:cropper],
           crop_w:a.instance.width,
           crop_x:a.instance.left,
           crop_y:a.instance.top}
      }
    },
    default_url: "/images/:style/user.png",
    validates_attachment_content_type :image, content_type: /\Aimage\/.*\z/
  </code></pre>
  
  <p>I can run <code>image.reprocess!</code>, but this can't be the right way, images would be processed twice.</p>