code: public function visit(Criterion $criterion, CriterionVisitor $subVisitor = null)
      {
          $condition = $this->escapeQuote(parent::visit($criterion, $subVisitor));
  
          return "{!child of='document_type_id:content' v='document_type_id:content AND {$condition}'}";
      }
docs: Map field value to a proper Solr representation.
  
  @param \eZ\Publish\API\Repository\Values\Content\Query\Criterion $criterion
  @param \EzSystems\EzPlatformSolrSearchEngine\Query\CriterionVisitor $subVisitor
  
  @return string