Make uniqueness constraints easy

[jacobobryant]

Example problem: right now, the template project has a race condition when creating users (see https://github.com/jacobobryant/biff/blob/master/libs/tasks/resources/biff/template/src/%7B%7Bparent-path%7D%7D/routes/auth.clj#L68-L78). In general, email address shouldn't be used as the primary key for a user document/record, since it can change. But we still want to enforce uniqueness.

This can be done by creating a separate document (call it a "constraint doc") and matching on it, e.g. {:crux.db/id {:user/email "foo@example.com"} :biff/held-by #uuid "some-doc-id"}.

First, we should do this in the project template without helper fns. This will only take a minute, since the project template doesn't let users change their email, and it doesn't delete users. I just don't want to do it right now since it's not urgent and I'm busy writing docs.

Second, we should think about abstracting this into a Biff lib. For example, maybe in the schema we add a key like this:

:unique-keys #{:user/email #{:foo/bar :foo/baz} ...}

And then have biff.crux/submit-tx add matches + create the separate constraint docs based on that.

We could use transaction fns instead of matching. Need to decide which is better. My initial whim is to use matching.

[jacobobryant]

Lately I'm starting to lean towards always using UUIDs for IDs on regular documents, and then relying on something like this for all uniqueness constraints.

[jacobobryant]

New version of biff includes a :db/lookup special value for this.