title: How to aggregate values into collection after groupBy?
body: <p>I have a dataframe with schema as such:</p>
  
  <pre><code>[visitorId: string, trackingIds: array&lt;string&gt;, emailIds: array&lt;string&gt;]
  </code></pre>
  
  <p>Looking for a way to group (or maybe rollup?) this dataframe by visitorid where the trackingIds and emailIds columns would append together.  So for example if my initial df looks like:</p>
  
  <pre><code>visitorId   |trackingIds|emailIds
  +-----------+------------+--------
  |a158|      [666b]      |    [12]
  |7g21|      [c0b5]      |    [45]
  |7g21|      [c0b4]      |    [87]
  |a158|      [666b, 777c]|    []
  </code></pre>
  
  <p>I would like my output df to look like this</p>
  
  <pre><code>visitorId   |trackingIds|emailIds
  +-----------+------------+--------
  |a158|      [666b,666b,777c]|      [12,'']
  |7g21|      [c0b5,c0b4]     |      [45, 87]
  </code></pre>
  
  <p>Attempting to use <code>groupBy</code> and <code>agg</code> operators but not have much luck.</p>