save_belongs_to_association

# File activerecord/lib/active_record/autosave_association.rb, line 494
      def save_belongs_to_association(reflection)
        association = association_instance_get(reflection.name)
        return unless association && association.loaded? && !association.stale_target?

        record = association.load_target
        if record && !record.destroyed?
          autosave = reflection.options[:autosave]

          if autosave && record.marked_for_destruction?
            self[reflection.foreign_key] = nil
            record.destroy
          elsif autosave != false
            saved = record.save(validate: !autosave) if record.new_record? || (autosave && record.changed_for_autosave?)

            if association.updated?
              association_id = record.public_send(reflection.options[:primary_key] || :id)
              self[reflection.foreign_key] = association_id
              association.loaded!
            end

            saved if autosave
          end
        end
      end