Fix ClassCastException when using dynamic template with flat_object mapping and dots in field names

CHANGELOG.md

@@ -138,6 +138,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix bulk API ignores ingest pipeline for upsert ([#12883](https://github.com/opensearch-project/OpenSearch/pull/12883))
 - Fix issue with feature flags where default value may not be honored ([#12849](https://github.com/opensearch-project/OpenSearch/pull/12849))
 - Fix UOE While building Exists query for nested search_as_you_type field ([#12048](https://github.com/opensearch-project/OpenSearch/pull/12048))
+- Fix ClassCastException when using dynamic template with flat_object mapping and dots in field names ([#13081](https://github.com/opensearch-project/OpenSearch/pull/13081))
 
 ### Security
 

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

@@ -936,7 +936,13 @@ private static Tuple<Integer, ObjectMapper> getDynamicParentMapper(
                             context.indexSettings().getSettings(),
                             context.path()
                         );
-                        mapper = (ObjectMapper) builder.build(builderContext);
+                        Mapper tmp = builder.build(builderContext);
+                        if (tmp instanceof FlatObjectFieldMapper) {
+                            throw new MapperParsingException(
+                                "It is forbidden to create dynamic flat_object ([" + String.join(".", paths) + "]) with dots in field names"
+                            );
+                        }
+                        mapper = (ObjectMapper) tmp;
                         if (mapper.nested() != ObjectMapper.Nested.NO) {
                             throw new MapperParsingException(
                                 "It is forbidden to create dynamic nested objects (["

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

@@ -217,6 +217,28 @@ public void testDotsWithDynamicNestedMapper() throws Exception {
         assertEquals("It is forbidden to create dynamic nested objects ([foo]) through `copy_to` or dots in field names", e.getMessage());
     }
 
+    public void testDotsWithDynamicFlatObjectMapper() throws Exception {
+        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
+            b.startArray("dynamic_templates");
+            {
+                b.startObject();
+                {
+                    b.startObject("nested_object_fields");
+                    {
+                        b.field("match_mapping_type", "object");
+                        b.startObject("mapping").field("type", "flat_object").endObject();
+                    }
+                    b.endObject();
+                }
+                b.endObject();
+            }
+            b.endArray();
+        }));
+
+        MapperParsingException e = expectThrows(MapperParsingException.class, () -> mapper.parse(source(b -> b.field("foo.bar", 42))));
+        assertEquals("It is forbidden to create dynamic flat_object ([foo.bar]) with dots in field names", e.getMessage());
+    }
+
     public void testNestedHaveIdAndTypeFields() throws Exception {
 
         DocumentMapper mapper = createDocumentMapper(mapping(b -> {