instance method save_belongs_to_association

# File activerecord/lib/active_record/autosave_association.rb, line 545
      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?
            foreign_key = ActiveRecord::Key.for(reflection.foreign_key)
            foreign_key.each { |key| write_attribute(key, nil) }
            record.destroy
          elsif autosave != false
            saved = if record.new_record? || (autosave && record.changed_for_autosave?)
              begin
                @autosaving_belongs_to_for ||= {}
                @autosaving_belongs_to_for[association] = true
                record.save(validate: !autosave)
              ensure
                @autosaving_belongs_to_for[association] = false
              end
            end

            if association.updated?
              primary_key = ActiveRecord::Key.for(reflection.association_primary_key(record.class))
              foreign_key = ActiveRecord::Key.for(reflection.foreign_key)

              primary_key_foreign_key_pairs = primary_key.zip(foreign_key)
              primary_key_foreign_key_pairs.each do |primary_key, foreign_key|
                association_id = record.read_attribute(primary_key)
                write_attribute(foreign_key, association_id) unless read_attribute(foreign_key) == association_id
              end
              association.loaded!
            end

            saved if autosave
          end
        end
      end