The docs says that ancestor will apply a filter of given ancestor being somewhere in the ancestry tree.
You can filter your datastore queries to a specified ancestor, so that results contain only entities containing that ancestor. In other words, all of the results will have the ancestor as their parent, or parent's parent, or etc. Passing None as a parameter does not query for entities without ancestors and will return errors.
Can I somehow filter on one level, immediate ancestor relationship so that a query will provide models having specific parent? Or must I store parent relationship in a ReferenceProperty and filter on that?