fix(json): Fix composed type (oneOf) serialization returning empty object

packages/serialization/json/kiota_serialization_json/json_serialization_writer.py

@@ -318,10 +318,13 @@ def write_object_value(
             if value and self._on_after_object_serialization:
                 self._on_after_object_serialization(value)
 
+            # Use temp_writer.value if available (for composed types like oneOf wrappers),
+            # otherwise fall back to temp_writer.writer (for regular objects with properties)
+            serialized_value = temp_writer.value if temp_writer.value is not None else temp_writer.writer
             if key:
-                self.writer[key] = temp_writer.writer
+                self.writer[key] = serialized_value
             else:
-                self.value = temp_writer.writer
+                self.value = serialized_value
 
     def write_enum_value(self, key: Optional[str], value: Optional[K]) -> None:
         """Writes the specified enum value to the stream with an optional given key.

packages/serialization/json/tests/unit/test_json_serialization_writer.py

@@ -282,3 +282,25 @@ def test_write_additional_data_value(user_1, user_2):
                 '{"display_name": "John Doe", "age": 32}], "created_at": "2022-01-27", '\
                 '"data": {"groups": [{"friends": [{"display_name": "John Doe", "age": 32}]}]}, '\
                     '"pinnedItems": [null, null]}'
+
+
+def test_write_composed_type_with_no_key(user_1):
+    """Test that composed types (oneOf/union types) serialize correctly when using write_object_value with None key.
+
+    This tests the fix for the bug where composed types that call write_object_value(None, inner_value)
+    would serialize to {} instead of the actual object content.
+    """
+    from ..helpers.union_type import UnionType
+
+    # Create a union type with a composed type inside
+    union = UnionType()
+    union.composed_type1 = user_1
+
+    json_serialization_writer = JsonSerializationWriter()
+    json_serialization_writer.write_object_value(None, union)
+    content = json_serialization_writer.get_serialized_content()
+    content_string = content.decode('utf-8')
+
+    # The union type should serialize the inner user object, not an empty object
+    assert content_string == '{"id": "8f841f30-e6e3-439a-a812-ebd369559c36", '\
+        '"updated_at": "2022-01-27T12:59:45.596117+00:00", "is_active": true}'