Question

From my understanding, @db.transactional(xg=True) allows for transactions across groups, however the following code returns “queries inside transactions must have ancestors”.

@db.transactional(xg=True)
def insertUserID(self,userName):
    user = User.gql("WHERE userName = :1", userName).get()
    highestUser = User.all().order('-userID').get()
    nextUserID = highestID + 1 
    user.userID = nextUserID
    user.put()

Do you need to pass in the key for each entity despite being a cross group transaction? Can you please help modify this example accordingly?

Answer 1

Cross group transactions allow you to perform a transaction across multiple groups, but they don’t remove the prohibition on queries inside transactions. You need to perform the query outside the transaction, and pass the ID of the entity in (and then check any invariants specified in the query still hold) – or, as Shay suggests, use IDs so you don’t have to do a query in the first place.