title: Aggregations for inner hits
body: <p>I have a parent-children mapping in ElasticSearch:</p>
  
  <p>parent: <em>user</em></p>
  
  <p>children: <em>privileges</em></p>
  
  <p>For <em>privileges</em> there are a few properties, and one is "<em>privilegeName</em>".</p>
  
  <p>The query returns users which have certain privileges, but I would like to return the aggregated <em>privilegeName</em>s for each user for the privileges that match the <code>has_child</code> query.
  I can return all <em>privileges</em> with <code>inner_hits</code> and process them on the client side, but that may be cumbersome.
  Is there a possibility to aggregate the <code>inner_hits</code>?</p>
  
  <p>Thanks</p>