private FilterDocument createAssociatedFeatureFilter(String featureType, String identifier) {
   if (associations.containsKey(featureType)) {
     AssociatedFeature associate = associations.get(featureType);
     FilterDocument doc =
         createFilterByValueReference(
             "wfs:valueOf(*/*/" + associate.getAssociatedFeatureElement() + ")/*/gml:identifier",
             identifier);
     return doc;
   }
   return null;
 }