- presence: true is redundant since Rails 5.0 BUT applies
with new default config of
belongs_to_required_by_default to true.
Lots of files with belongs_to_required_by_default = false
(backward compatibility).
So: deleting this setting implies to adding optional: true
It would take ages to go through all files now and assess all belongs_to
associations. So I just declare the old default and then we can move on
and apply the new default for the application while these classes still
use the old one. All new models will then use the new default which is
the goal of this excercise and we can refactor old classes when we touch
them anyway.
