[Backport 2.x] Fix exists queries on nested flat_object fields throws exception

CHANGELOG.md

@@ -44,6 +44,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix case insensitive and escaped query on wildcard ([#16827](https://github.com/opensearch-project/OpenSearch/pull/16827))
 - Fix illegal argument exception when creating a PIT ([#16781](https://github.com/opensearch-project/OpenSearch/pull/16781))
 - Fix NPE in node stats due to QueryGroupTasks ([#17576](https://github.com/opensearch-project/OpenSearch/pull/17576))
+- 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

@@ -16,7 +16,10 @@ setup:
                 type : "flat_object"
               required_matches:
                 type : "long"
-
+              infos:
+                properties:
+                  info:
+                    type: "flat_object"
   - do:
       index:
         index: test
@@ -60,7 +63,12 @@ setup:
             "review": [["bad",30.41],["ok",80.0]],
             "publishDate": "2016-01-01"
           },
-          "required_matches": 1
+          "required_matches": 1,
+          "infos": {
+            "info":{
+              "name": "name1"
+            }
+          }
         }
   # Do index refresh
   - do:
@@ -73,6 +81,40 @@ 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  }
+
+  - do:
+      search:
+        body: {
+          _source: true,
+          size: 10,
+          query: {
+            exists: {
+              field: "infos"
+            }
+          }
+        }
+  - length:   { hits.hits: 1  }
+  - match: { hits.hits.0._source.infos.info.name: "name1" }
+
 ---
 "Invalid docs":
   - skip:
@@ -135,7 +177,7 @@ teardown:
   - match: { test.mappings.properties.catalog.type: flat_object }
   - match: { test.mappings.properties.required_matches.type: long }
   # https://github.com/opensearch-project/OpenSearch/tree/main/rest-api-spec/src/main/resources/rest-api-spec/test#length
-  - length: { test.mappings.properties: 3 }
+  - length: { test.mappings.properties: 4 }
   - length: { test.mappings.properties.catalog: 1 }
 
 ---

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

@@ -93,7 +93,12 @@ public static class Defaults {
 
     @Override
     public MappedFieldType keyedFieldType(String key) {
-        return new FlatObjectFieldType(this.name() + DOT_SYMBOL + key, this.name(), valueFieldType, valueAndPathFieldType);
+        return new FlatObjectFieldType(
+            Strings.isNullOrEmpty(key) ? this.name() : (this.name() + DOT_SYMBOL + key),
+            this.name(),
+            valueFieldType,
+            valueAndPathFieldType
+        );
     }
 
     /**
@@ -177,7 +182,7 @@ public FlatObjectFieldType(
                 new TextSearchInfo(Defaults.FIELD_TYPE, null, Lucene.KEYWORD_ANALYZER, Lucene.KEYWORD_ANALYZER),
                 Collections.emptyMap()
             );
-            assert rootFieldName == null || (name.length() > rootFieldName.length() && name.startsWith(rootFieldName));
+            assert rootFieldName == null || (name.length() >= rootFieldName.length() && name.startsWith(rootFieldName));
             this.ignoreAbove = Integer.MAX_VALUE;
             this.nullValue = null;
             this.rootFieldName = rootFieldName;

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

@@ -17,6 +17,7 @@
 import org.apache.lucene.search.TermQuery;
 import org.apache.lucene.util.BytesRef;
 import org.opensearch.common.TriFunction;
+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;
@@ -27,6 +28,7 @@
 
 import java.io.IOException;
 import java.util.List;
+import java.util.Set;
 
 import static org.opensearch.index.mapper.FlatObjectFieldMapper.CONTENT_TYPE;
 import static org.opensearch.index.mapper.FlatObjectFieldMapper.VALUE_AND_PATH_SUFFIX;
@@ -397,7 +399,17 @@ 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(b -> b.startObject("properties").startObject("foo").field("type", "flat_object").endObject().endObject())
+        );
 
+        QueryShardContext queryShardContext = createQueryShardContext(mapperService);
+        Set<String> fields = queryShardContext.simpleMatchToIndexNames("field.*");
+        assertEquals(1, fields.size());
+        assertEquals(Sets.newHashSet("field.foo"), fields);
     }
 
     @Override