I'm catching this thread late but I'd create a model whose parent entity would be an instance of your service model. This new model would be sharded to reduce contention and it would contain a list of all the customer id's who are subscribed to the service.
You can reduce contention even more by assigning meaning to + and - and append these to the customer ids in the sharded intances' lists. Customer id + '+' would indicate subscribing to the service whereas Customer id + '-' would indicate their unsubscribing from the service. You can query on Cursomer id + '+' and Customer id + '-' and which ever has a greater count tells you if they are subscribed or unsubscribed and no results tell you that they never subscribed. This technique removes the need to update entries in the list which can be time consuming and contentious. Just my $0.02 Jeff On Wed, Jul 21, 2010 at 1:50 PM, Jason C <[email protected]> wrote: > Weird, I was _just_ discussing this with a colleague at our office. > > If you want a really hard check and are willing to pay some overhead > for it, AND you can't use the key_name approach that Robert suggests > which is the best solution, you can use a technique like this: > > http://squeeville.com/2009/01/30/add-a-unique-constraint-to-google-app-engine/ > > We've typically used this for a User entity where we need a unique > email address, but we don't want to drive the email address into the > key_name because email addresses change. > > j > > On Jul 20, 9:42 pm, cwaldbieser <[email protected]> wrote: > > I have 3 models: A customer account, a service, and a model that > > represents a customer's subscription to the service. The subscription > > is essentially: > > > > class Subscription(db.Model): > > customer = db.ReferenceProperty(Customer) > > service = db.ReferenceProperty(Service) > > > > I have a user interface that allows creating or cancelling a > > customer's subscription. I want to be able to create a subscription > > only if an existing subscription does not already exist. I thought > > that a transaction might be the key, but I can't do a query in > > transaction unless it is by key or an ancestor query. So I am not > > sure what to do. > > > > Any thoughts? > > > > Thanks, > > Carl > > -- > You received this message because you are subscribed to the Google Groups > "Google App Engine" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]<google-appengine%[email protected]> > . > For more options, visit this group at > http://groups.google.com/group/google-appengine?hl=en. > > -- -- Jeff -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
