title: Use Ransack sort_link for non-attributes
body: <p>I'm using Ransack's sort_link in my rails project to display a list of fees for my <em>payment</em> model. However, 'fee_amount' isn't an attribute of the <em>payment</em> model, but instead a class method of the <em>bill</em> model (which belongs to payment). What I have currently is:</p>
  
  <blockquote>
    <p>&lt;%= sort_link @search, :bill_fee_amount, "Convenience Fee" %></p>
  </blockquote>
  
  <p>which should accesses the <em>bill</em> of the current <em>payment</em> and call the 'fee_amount' method on that <em>bill</em>, which does some calculations and returns a float.  It's these floats that I'm hoping to search by.</p>
  
  <p>Can this be done, or can I only sort by attributes of the model I'm dealing with?</p>