For example, provide hook method

Indeed, yup. Also really we'll want to make sure that any uniqueness errors are included alongside any other errors, rather than only running uniqueness validation once everything else passes.

Will need to think about this one a bit.

Plus we'll want to include the same validation on update too, but excluding pk=instance.pk

@tomchristie I added uniqueness validation in .update() too. The way I did it was to create a subclass of typesystem.Object which allows to see how the "hook" thing could look. :-) All errors are now returned as one exception too.

Hi @tomchristie, I realize this PR has been around for a while — are there any blockers here? I suppose the fact that this PR works around TypeSystem not providing async validation may be one. Happy to receive any pointers on how to tackle/clean this up, either here on in TypeSystem.