I think you are also conflating the issue of inconsistent queries with safe updates of the data.
A query like the one in your example UserGroupData.query(UserGroupData.users_in_group==user_id).get()
will always only return one entity, if the user_id is in the group.
If it has only just been added and the index is not up to date then you won't get a record and therefore you won't update the record.
Any update irrespective of the method of fetching the entity should be performed inside a transaction ensuring update consistency.
As to ancestors improving the consistency of the query, it's not obvious if you plan to have multiple UserGroupData entities. In which case why are you doing a get().
So option 3, is probably your best bet, do the keys only query, then inside a transaction do the Key.get() and update. Remember cross group transactions are limited 5 entity groups.
Given this approach if the index the query is based is out of date then 1 of 3 things can happen,
- the record you want isn't found because the newly added userid is not reflected in the index.
- the record you want is found, the get() will fetch it consistently
- the record you want is found, but the userid has actually been removed and the index is out of date. The get() will retrieve the index consistently and the userid is not present.
You code can then decide what course of action.
What is the use case for querying all UserGroupData entities that a particular user is a member of that would require updates ?