Add unit test for serialization of derived fields from query

Signed-off-by: Rishabh Maurya <rishabhmaurya05@gmail.com>

Author: rishabhmaurya

server/src/main/java/org/opensearch/search/builder/SearchSourceBuilder.java

@@ -1570,14 +1570,15 @@ public XContentBuilder innerToXContent(XContentBuilder builder, Params params) t
         if (derivedFieldsObject != null || derivedFields != null) {
             builder.startObject(DERIVED_FIELDS_FIELD.getPreferredName());
             if (derivedFieldsObject != null) {
-                builder.map(derivedFieldsObject);
+                builder.mapContents(derivedFieldsObject);
             }
             if (derivedFields != null) {
                 for (DerivedField derivedField : derivedFields) {
                     derivedField.toXContent(builder, params);
                 }
             }
             builder.endObject();
+
         }
 
         return builder;

server/src/test/java/org/opensearch/search/builder/SearchSourceBuilderTests.java

@@ -53,6 +53,7 @@
 import org.opensearch.index.query.QueryRewriteContext;
 import org.opensearch.index.query.RandomQueryBuilder;
 import org.opensearch.index.query.Rewriteable;
+import org.opensearch.script.Script;
 import org.opensearch.search.AbstractSearchTestCase;
 import org.opensearch.search.rescore.QueryRescorerBuilder;
 import org.opensearch.search.sort.FieldSortBuilder;
@@ -311,7 +312,7 @@ public void testParseSort() throws IOException {
         }
     }
 
-    public void testDerivedFieldsParsing() throws IOException {
+    public void testDerivedFieldsParsingAndSerialization() throws IOException {
         {
             String restContent = "{\n"
                 + "  \"derived\": {\n"
@@ -328,10 +329,29 @@ public void testDerivedFieldsParsing() throws IOException {
                 + "        \"match\": { \"content\": { \"query\": \"foo bar\" }}\n"
                 + "     }\n"
                 + "}";
+
+            String expectedContent =
+                "{\"query\":{\"match\":{\"content\":{\"query\":\"foo bar\",\"operator\":\"OR\",\"prefix_length\":0,\"max_expansions\":50,\"fuzzy_transpositions\":true,\"lenient\":false,\"zero_terms_query\":\"NONE\",\"auto_generate_synonyms_phrase_query\":true,\"boost\":1.0}}},"
+                    + "\"derived\":{"
+                    + "\"duration\":{\"type\":\"long\",\"script\":\"emit(doc['test'])\"},\"ip_from_message\":{\"type\":\"keyword\",\"script\":\"emit(doc['message'])\"},\"derived_field\":{\"type\":\"keyword\",\"script\":{\"source\":\"emit(doc['message']\",\"lang\":\"painless\"}}}}";
+
             try (XContentParser parser = createParser(JsonXContent.jsonXContent, restContent)) {
                 SearchSourceBuilder searchSourceBuilder = SearchSourceBuilder.fromXContent(parser);
+                searchSourceBuilder.derivedField("derived_field", "keyword", new Script("emit(doc['message']"));
                 searchSourceBuilder = rewrite(searchSourceBuilder);
                 assertEquals(2, searchSourceBuilder.getDerivedFieldsObject().size());
+                assertEquals(1, searchSourceBuilder.getDerivedFields().size());
+
+                try (BytesStreamOutput output = new BytesStreamOutput()) {
+                    searchSourceBuilder.writeTo(output);
+                    try (StreamInput in = new NamedWriteableAwareStreamInput(output.bytes().streamInput(), namedWriteableRegistry)) {
+                        SearchSourceBuilder deserializedBuilder = new SearchSourceBuilder(in);
+                        String actualContent = deserializedBuilder.toString();
+                        assertEquals(expectedContent, actualContent);
+                        assertEquals(searchSourceBuilder.hashCode(), deserializedBuilder.hashCode());
+                        assertNotSame(searchSourceBuilder, deserializedBuilder);
+                    }
+                }
             }
         }