NonNull Violation and calling nextval() on sequence_name when primary key is uuid

[Amnesthesia]

When primary key is of type uuid, we're getting a non-null violation for column ID (using postgres) especially when using on_duplicate_key_update, but only when some records in the array have an ID.

We've worked around this by splitting our imports so that it imports records that need to be updated separately from records that need to be imported, and it works. When some records have IDs and others dont, we get non-null violations on ID columns. This doesn't normally happen if ID is an integer — and why sequence_name is set on the model when there's no sequence for this primary key, I have no idea.