title: How to join with dataset with column as the collection of keys to join by?
body: <p>I'm trying to figure out how to join two Dataframes where one of the dataframes contains a list that has the key to join by. </p>
  
  <p>For example:</p>
  
  <ul>
  <li>one dataframe has the format <code>(id: String, ...,...)</code> </li>
  <li>the other has <code>(... ,..., List[tmpObj])</code></li>
  <li><code>tmpObj</code> is structured like this <code>(id: String, value: int)</code>.</li>
  </ul>
  
  <p>My goal is to join the two dataframes by the id values found in the second dataFrame's list of tmpObj to the id from the first dataframe. </p>
  
  <p>Does anyone have any idea of how to go about this?</p>