diff --git a/odmantic/model.py b/odmantic/model.py
index 08308f29..13fdeb02 100644
--- a/odmantic/model.py
+++ b/odmantic/model.py
@@ -609,7 +609,7 @@ def _post_copy_update(self: BaseT) -> None:
 
         Set them as if they were modified to make sure they are saved in the database.
         """
-        object.__setattr__(self, "__fields_modified__", set(self.model_fields))
+        object.__setattr__(self, "__fields_modified__", set(self.__class__.model_fields))
         for field_name, field in self.__odm_fields__.items():
             if isinstance(field, ODMEmbedded):
                 value = getattr(self, field_name)
@@ -1006,7 +1006,7 @@ def model_update(
     ) -> None:
         is_primary_field_in_patch = (
             isinstance(patch_object, BaseModel)
-            and self.__primary_field__ in patch_object.model_fields
+            and self.__primary_field__ in patch_object.__class__.model_fields
         ) or (isinstance(patch_object, dict) and self.__primary_field__ in patch_object)
         if is_primary_field_in_patch:
             if (