Fix exists queries on nested flat_object fields throws exception

CHANGELOG.md

@@ -97,6 +97,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix Shallow copy snapshot failures on closed index ([#16868](https://github.com/opensearch-project/OpenSearch/pull/16868))
 - Fix multi-value sort for unsigned long ([#16732](https://github.com/opensearch-project/OpenSearch/pull/16732))
 - The `phone-search` analyzer no longer emits the tel/sip prefix, international calling code, extension numbers and unformatted input as a token ([#16993](https://github.com/opensearch-project/OpenSearch/pull/16993))
+- Fix exists queries on nested flat_object fields throws exception ([#16803](https://github.com/opensearch-project/OpenSearch/pull/16803))
 
 ### Security
 

rest-api-spec/src/main/resources/rest-api-spec/test/index/90_flat_object.yml

@@ -73,6 +73,26 @@ teardown:
   - do:
       indices.delete:
         index: test
+
+---
+"Exist query in root field":
+  - skip:
+      version: "- 2.99.99"
+      reason: "the query would throw exception prior to 2.99.99"
+
+  - do:
+      search:
+        body: {
+          _source: true,
+          size: 10,
+          query: {
+            exists: {
+              field: "catalog"
+            }
+          }
+        }
+  - length:   { hits.hits: 2  }
+
 ---
 "Invalid docs":
   - skip:

server/src/main/java/org/opensearch/index/mapper/FlatObjectFieldMapper.java

@@ -27,6 +27,7 @@
 import org.opensearch.common.unit.Fuzziness;
 import org.opensearch.common.xcontent.JsonToStringXContentParser;
 import org.opensearch.core.common.ParsingException;
+import org.opensearch.core.common.Strings;
 import org.opensearch.core.common.io.stream.StreamOutput;
 import org.opensearch.core.xcontent.DeprecationHandler;
 import org.opensearch.core.xcontent.NamedXContentRegistry;
@@ -87,7 +88,7 @@ public static class Defaults {
     @Override
     public MappedFieldType keyedFieldType(String key) {
         return new FlatObjectFieldType(
-            this.name() + DOT_SYMBOL + key,
+            Strings.isNullOrEmpty(key) ? this.name() : (this.name() + DOT_SYMBOL + key),
             this.name(),
             (KeywordFieldType) valueFieldMapper.fieldType(),
             (KeywordFieldType) valueAndPathFieldMapper.fieldType()

server/src/test/java/org/opensearch/index/mapper/FlatObjectFieldMapperTests.java

@@ -16,6 +16,7 @@
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.TermQuery;
 import org.apache.lucene.util.BytesRef;
+import org.opensearch.common.util.set.Sets;
 import org.opensearch.common.xcontent.XContentFactory;
 import org.opensearch.common.xcontent.json.JsonXContent;
 import org.opensearch.core.xcontent.ToXContent;
@@ -24,6 +25,7 @@
 import org.opensearch.search.DocValueFormat;
 
 import java.io.IOException;
+import java.util.Set;
 
 import static org.opensearch.common.xcontent.JsonToStringXContentParser.VALUE_AND_PATH_SUFFIX;
 import static org.opensearch.common.xcontent.JsonToStringXContentParser.VALUE_SUFFIX;
@@ -416,7 +418,14 @@ public void testFetchDocValues() throws IOException {
             Throwable throwable = assertThrows(IllegalArgumentException.class, () -> ft.docValueFormat(null, null));
             assertEquals("Field [field] of type [flat_object] does not support doc_value in root field", throwable.getMessage());
         }
+    }
 
+    public void testPatternMatch() throws IOException {
+        MapperService mapperService = createMapperService(fieldMapping(this::minimalMapping));
+        QueryShardContext queryShardContext = createQueryShardContext(mapperService);
+        Set<String> fields = queryShardContext.simpleMatchToIndexNames("field.*");
+        assertEquals(2, fields.size());
+        assertEquals(Sets.newHashSet("field._value", "field._valueAndPath"), fields);
     }
 
     @Override