Best practice about empty belongs_to association

Be careful with this in Rails 5…

#belongs_to is required by default

From now on every Rails application will have a new configuration
option config.active_record.belongs_to_required_by_default = true, it
will trigger a validation error when trying to save a model where
belongs_to associations are not present.

config.active_record.belongs_to_required_by_default can be changed to
false and with this keep old Rails behavior or we can disable this
validation on each belongs_to definition, just passing an additional
option optional: true as follows:

class Book < ActiveRecord::Base
  belongs_to :author, optional: true
end

from: https://sipsandbits.com/2015/09/21/whats-new-in-rails-5/#belongs_toisrequiredbydefault