I don't know preferred style, but we can find both, ex) [here](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/nested_attributes.rb#L382) and [here](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/reflection.rb#L445) etc.
I will wait for responses from others. Thanks!