I think it should be like this, to mirror the behaviour of [Association#build_record](https://github.com/exviva/rails/blob/2d2cbf22c9ef385595b4a01dd4947da90a0158f9/activerecord/lib/active_record/associations/association.rb#L241). Actually, not the whole `create_scope` is assigned - only columns that didn't change, foreign key, and potentially STI type column.

The second test is showing that the foreign key cannot be nested-mass-assigned to a new, in-memory record.