title: Scalding: Fill in Discontinuity in Rows
body: <p>Let's pretend I have data like this:</p>
  
  <pre><code>(Date, Most Active User)
  (6/1/2014, "Bob")
  (6/2/2014, "Joe")
  (6/3/2014, "Jim")
  (6/7/2014, "Jack")
  </code></pre>
  
  <p>Notice there are missing rows for date (6/4/2014), (6/5/2014), (6/6/2014). I want to fill in the default "Most Active User" value for those rows to the most recent row where that value is defined. For example, the value for those rows should be "Jim". </p>