title: Using accepts_nested_attributes_for with rejects_if
body: <p>I'm using paperclip to attach photos to one of my models:</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    has_many :photos
    accepts_nested_attributes_for :photos
  end
  
  class Photo &lt; ActiveRecord::Base
    belongs_to :user
    has_attached_file :data
  end
  </code></pre>
  
  <p>How can I use reject_if to ignore data fields to which files are not uploaded by users?</p>