Small fixes to flattened field value fetching. (#63443)

* Remove FlatObjectFieldTypeTests, as it's redundant.
* Do not apply null_value when fetching root-level values.
* Remove a TODO in favor of opening an issue.

Author: jtibshirani

x-pack/plugin/mapper-flattened/src/main/java/org/elasticsearch/xpack/flattened/mapper/FlatObjectFieldMapper.java

@@ -177,8 +177,8 @@ public Builder store(boolean store) {
 
         @Override
         public FlatObjectFieldMapper build(BuilderContext context) {
-            MappedFieldType ft
-                = new RootFlatObjectFieldType(buildFullName(context), indexed, hasDocValues, meta, splitQueriesOnWhitespace, nullValue);
+            MappedFieldType ft = new RootFlatObjectFieldType(
+                buildFullName(context), indexed, hasDocValues, meta, splitQueriesOnWhitespace);
             if (eagerGlobalOrdinals) {
                 ft.setEagerGlobalOrdinals(true);
             }
@@ -323,7 +323,7 @@ public IndexFieldData.Builder fielddataBuilder(String fullyQualifiedIndexName, S
 
         @Override
         public ValueFetcher valueFetcher(MapperService mapperService, SearchLookup searchLookup, String format) {
-            throw new UnsupportedOperationException();  // TODO can we implement this?
+            throw new UnsupportedOperationException();
         }
     }
 
@@ -438,15 +438,13 @@ public IndexFieldData<?> build(IndexFieldDataCache cache, CircuitBreakerService
      */
     public static final class RootFlatObjectFieldType extends StringFieldType {
         private final boolean splitQueriesOnWhitespace;
-        private final String nullValue;
 
         public RootFlatObjectFieldType(String name, boolean indexed, boolean hasDocValues, Map<String, String> meta,
-                                       boolean splitQueriesOnWhitespace, String nullValue) {
+                                       boolean splitQueriesOnWhitespace) {
             super(name, indexed, false, hasDocValues,
                 splitQueriesOnWhitespace ? TextSearchInfo.WHITESPACE_MATCH_ONLY : TextSearchInfo.SIMPLE_MATCH_ONLY, meta);
             this.splitQueriesOnWhitespace = splitQueriesOnWhitespace;
             setIndexAnalyzer(Lucene.KEYWORD_ANALYZER);
-            this.nullValue = nullValue;
         }
 
         @Override
@@ -474,7 +472,7 @@ public ValueFetcher valueFetcher(MapperService mapperService, SearchLookup searc
             if (format != null) {
                 throw new IllegalArgumentException("Field [" + name() + "] of type [" + typeName() + "] doesn't support formats.");
             }
-            return new SourceValueFetcher(name(), mapperService, nullValue) {
+            return new SourceValueFetcher(name(), mapperService) {
                 @Override
                 protected Object parseSourceValue(Object value) {
                     return value;

x-pack/plugin/mapper-flattened/src/test/java/org/elasticsearch/xpack/flattened/mapper/FlatObjectFieldTypeTests.java

@@ -22,12 +22,15 @@
 import org.elasticsearch.index.mapper.FieldTypeTestCase;
 import org.elasticsearch.xpack.flattened.mapper.FlatObjectFieldMapper.RootFlatObjectFieldType;
 
+import java.io.IOException;
 import java.util.Collections;
+import java.util.List;
+import java.util.Map;
 
 public class RootFlatObjectFieldTypeTests extends FieldTypeTestCase {
 
     private static RootFlatObjectFieldType createDefaultFieldType() {
-        return new RootFlatObjectFieldType("field", true, true, Collections.emptyMap(), false, null);
+        return new RootFlatObjectFieldType("field", true, true, Collections.emptyMap(), false);
     }
 
     public void testValueForDisplay() {
@@ -49,19 +52,19 @@ public void testTermQuery() {
 
 
         RootFlatObjectFieldType unsearchable = new RootFlatObjectFieldType("field", false, true,
-            Collections.emptyMap(), false, null);
+            Collections.emptyMap(), false);
         IllegalArgumentException e = expectThrows(IllegalArgumentException.class,
             () -> unsearchable.termQuery("field", null));
         assertEquals("Cannot search on field [field] since it is not indexed.", e.getMessage());
     }
 
     public void testExistsQuery() {
-        RootFlatObjectFieldType ft = new RootFlatObjectFieldType("field", true, false, Collections.emptyMap(), false, null);
+        RootFlatObjectFieldType ft = new RootFlatObjectFieldType("field", true, false, Collections.emptyMap(), false);
         assertEquals(
             new TermQuery(new Term(FieldNamesFieldMapper.NAME, new BytesRef("field"))),
             ft.existsQuery(null));
 
-        RootFlatObjectFieldType withDv = new RootFlatObjectFieldType("field", true, true, Collections.emptyMap(), false, null);
+        RootFlatObjectFieldType withDv = new RootFlatObjectFieldType("field", true, true, Collections.emptyMap(), false);
         assertEquals(new DocValuesFieldExistsQuery("field"), withDv.existsQuery(null));
     }
 
@@ -122,4 +125,12 @@ public void testWildcardQuery() {
         assertEquals("[wildcard] queries cannot be executed when 'search.allow_expensive_queries' is set to false.",
                 ee.getMessage());
     }
+
+    public void testFetchSourceValue() throws IOException {
+        Map<String, Object> sourceValue = Map.of("key", "value");
+        RootFlatObjectFieldType ft = createDefaultFieldType();
+
+        assertEquals(List.of(sourceValue), fetchSourceValue(ft, sourceValue));
+        assertEquals(List.of(), fetchSourceValue(ft, null));
+    }
 }