save_has_one_association

# File activerecord/lib/active_record/autosave_association.rb, line 463
      def save_has_one_association(reflection)
        association = association_instance_get(reflection.name)
        return unless association && association.loaded?

        record = association.load_target
        return unless 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) }
          return unless (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|
              association_id = _read_attribute(primary_key)
              record[foreign_key] = association_id unless record[foreign_key] == association_id
            end
            association.set_inverse_instance(record)
          end

          inverse_association = reflection.inverse_of && record.association(reflection.inverse_of.name)
          return if inverse_association && record.autosaving_belongs_to_for?(inverse_association)

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