Raise error if save_condition fails #991

mongoengine/document.py

@@ -266,7 +266,8 @@ class Document(BaseDocument):
             to cascading saves.  Implies ``cascade=True``.
         :param _refs: A list of processed references used in cascading saves
         :param save_condition: only perform save if matching record in db
-            satisfies condition(s) (e.g., version number)
+            satisfies condition(s) (e.g., version number).
+            Raises :class:`OperationError` if the conditions are not satisfied
 
         .. versionchanged:: 0.5
             In existing documents it only saves changed fields using
@@ -348,6 +349,9 @@ class Document(BaseDocument):
                     upsert = save_condition is None
                     last_error = collection.update(select_dict, update_query,
                                                    upsert=upsert, **write_concern)
+                    if save_condition is not None and last_error['nModified'] == 0:
+                        raise OperationError('Race condition preventing'
+                                             ' document update detected')
                     created = is_new_object(last_error)
 
             if cascade is None: