title: Filtering a nested PySpark DataFrame based on the internal fields
body: <p>I have a DataFrame whose sample element is (obtained with a take(1)):</p>
  
  <pre><code>[Row(id=u'1', objects=Row('a'=Row(fav=True, ratio=0.5), 'b'=Row(fav=False, ratio=0.0))]
  </code></pre>
  
  <p>that is, the structure is composite so that the column <em>objects</em> is an array of Rows (which are then composite as well as they contain two fields). </p>
  
  <p>How would I filter based on the nested elements, namely on the content of objects? Say I want to search, for the row whose id is '1', which is the ratio on the object called 'b', for example?</p>