Adapt nested fields extraction from "fields" API output to the new un-flattened structure (#68745)

Author: astefan

x-pack/plugin/ql/src/main/java/org/elasticsearch/xpack/ql/execution/search/extractor/AbstractFieldHitExtractor.java

@@ -17,6 +17,7 @@
 import java.io.IOException;
 import java.time.ZoneId;
 import java.util.List;
+import java.util.Map;
 import java.util.Objects;
 
 /**
@@ -78,7 +79,13 @@ public void writeTo(StreamOutput out) throws IOException {
     @Override
     public Object extract(SearchHit hit) {
         Object value = null;
-        DocumentField field = hit.field(fieldName);
+        DocumentField field = null;
+        if (hitName != null) {
+            // a nested field value is grouped under the nested parent name (ie dep.dep_name lives under "dep":[{dep_name:value}])
+            field = hit.field(hitName);
+        } else {
+            field = hit.field(fieldName);
+        }
         if (field != null) {
             value = unwrapFieldsMultiValue(field.getValues());
         }
@@ -89,6 +96,10 @@ protected Object unwrapFieldsMultiValue(Object values) {
         if (values == null) {
             return null;
         }
+        if (values instanceof Map && hitName != null) {
+            // extract the sub-field from a nested field (dep.dep_name -> dep_name)
+            return unwrapFieldsMultiValue(((Map<?,?>) values).get(fieldName.substring(hitName.length() + 1)));
+        }
         if (values instanceof List) {
             List<?> list = (List<?>) values;
             if (list.isEmpty()) {