Type Error on Unsafe Upsert Functions

[parsonsmatt]

There are a couple of unsafe functions in persistent. It would be nice to fix them.

upsert fails at runtime if there isn't exactly one uniqueness constraint. We could have a type class UniqueUnique (or, probably better named, OnlyOneUnique) with valid instances being generated for entities that possess exactly one key. Other entities would instead have TypeError-laden instances. This moves failure from run-time to compile-time with very little cost to end users.