title: Friendly_id: In a model how to slug a second column value when value in first column is absent
body: <p>I am using Ruby 2.1.1 and Rails 4.0.4.</p>
  
  <p>In my model I have two columns A &amp; B. </p>
  
  <p>How do I slug value of B when value in column A is absent? </p>
  
  <p>I want to keep column A value primarily. It is only when the value in column A is absent.</p>