However, I think this is a good, and common, example as people’s email address changes over time, and that doesn’t mean the Account should have to change. A similar example is how you can adjust your Twitter screen_name (which is your login/username), but the rest of the Account is unchanged.

Something that isn’t mentioned in this post is that if you do allow someone to change this value (e.g., change their email on the Account), you have to explicitly remove the old email from Unique – that is, if you want it to be able to to be used again in the future.

I think I would use this solution for mutable data, which is not part of the primary (since primary keys should not be mutable anyhow).

A caveat of this is that if your a.put() fails, you’ve lost your unique value unless you take some other compensating action (e.g., removing the unique value from the Unique model).