How can I create two unique, queriable fields for a GAE Datastore Data Model?

Question

First a little setup. Last week I was having trouble implementing a specific methodology that I had constructed which would allow me to manage two unique fields associated wit

Answer 1

The system you describe will work, at the cost of transactionality. Note that the second entity is no longer a child entity - it's just another entity with a ReferenceProperty.

This solution may be sufficient to your needs - for instance, if you need to enforce that every user has a unique email address, but this is not your primary identifier for a user, you can insert a record into an 'emails' table first, then if that succeeds, insert your primary record. If a failure occurs after the first operation but before the second, you have an email address associated with no record. You can simply ignore this, or timestamp the record and allow it to be reclaimed after some period of time (for example, 30 seconds, the maximum length of a frontend request).

If your requirements on transactionality and uniqueness are stronger than that, there are other options with increasing levels of complexity, such as implementing some form of distributed transactions, but it's unlikely you'll actually need that. If you can tell us more about the nature of the records and the unique keys, we may be able to provide more detailed suggestions.