title: Rails 4, Paperclip and polymorphic association
body: <p>I have 2 models: News and Uploadedfile</p>
  
  <pre><code>class News &lt; ActiveRecord::Base
      has_many :uploadedfiles, as: :parent
      attr_accessible :title, :content, :author
  end
  
  class Uploadedfile &lt; ActiveRecord::Base
      belongs_to :parent, polymorphic: true
      has_attached_file :url
      attr_accessible :url_file_name, :url_content_type, :url_file_size, :url_updated_at
  end
  </code></pre>
  
  <p>And form:</p>
  
  <pre><code>&lt;%= form_for(@news) do |f| %&gt;
  
      &lt;div class="field"&gt;
  
          &lt;%= f.fields_for :uploadedfile, f.uploadedfile.new do |uf| %&gt;
              &lt;%= uf.label :url %&gt;&lt;br&gt;
              &lt;%= uf.file_field :url %&gt;
          &lt;% end %&gt;
  
      &lt;/div&gt;
  
    &lt;div class="actions"&gt;
      &lt;%= f.submit %&gt;
    &lt;/div&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>When i'm submitting form, my table <code>uploadedfile</code> is not changed</p>
  
  <p>where is the problem? thank you!</p>