title: How can I sort a SOLR search on a Child Field
body: <p>I am currently trying to sort a SOLR search based on a numeric value in a child document field.
  I have recently learned that SOLR 6.6 has added support for this via a function query called childfield().</p>
  
  <p>Can anyone please provide me an example of how to use this function properly? The documentation seems to be lacking and I keep getting an error that a sort order cannot be determined.
  Preferably an explanation/example of the $bjq parameter usage would help also.</p>