save_has_one_association

# File activerecord/lib/active_record/autosave_association.rb, line 442
      def save_has_one_association(reflection)
        association = association_instance_get(reflection.name)
        record      = association && association.load_target

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

          if autosave && record.marked_for_destruction?
            record.destroy
          elsif autosave != false
            primary_key = Array(compute_primary_key(reflection, self)).map(&:to_s)
            primary_key_value = primary_key.map { |key| _read_attribute(key) }

            if (autosave && record.changed_for_autosave?) || _record_changed?(reflection, record, primary_key_value)
              unless reflection.through_reflection
                foreign_key = Array(reflection.foreign_key)
                primary_key_foreign_key_pairs = primary_key.zip(foreign_key)

                primary_key_foreign_key_pairs.each do |primary_key, foreign_key|
                  record[foreign_key] = _read_attribute(primary_key)
                end
                association.set_inverse_instance(record)
              end

              saved = record.save(validate: !autosave)
              raise ActiveRecord::Rollback if !saved && autosave
              saved
            end
          end
        end
      end