Thanks Mark. I hadn't realised that in datomic using a unique/identity can be a stand in for using temp ids to link entities inserted in the same transaction, cool. In the docs unique identity seemed to me mainly sold as enabling "upserts", but "upserting within a transaction" like this makes more sense to me as being useful/sound than doing so outside of one, at least that's my take on it at the moment. P.S. This is one of the best practical presentations on datomic I've seen and I've watched many.
There's an important difference between :db.unique/value and :db.unique/identity and that is that :db.unique/identity implies upsert behavior. :db.unique/value does not. This is somewhat glossed over but it has a rather important implication. If you have an attribute with :db.unique/identity an transaction will either insert/update that entity, while a :db.unique/value will fail, i.e. prevent the transaction. If you want to enforce for example a unique account name throughout your database you can't simply slap :db.unique/identity on it and expect it to work. You need to do additional concurrency control with CAS.
