Inherited models creating NOT NULL restrictions on their parents

to reproduce, create the yaml included in sample.yaml (Note the not-null columns in each subclass)

if you try to save an OnlineProduct without setting copyright_year, or if you create a print_product without setting website_branding_id, then you will get a not null error.
Inherited tables should be able to set their own NOT NULL columns without affecting other tables in the inheritance tree (should be handled in the preSave() method? or is there another mechanism?)