save_belongs_to_association

# File activerecord/lib/active_record/autosave_association.rb, line 502
      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 = Array(reflection.foreign_key)
            foreign_key.each { |key| self[key] = nil }
            record.destroy
          elsif autosave != false
            saved = record.save(validate: !autosave) if record.new_record? || (autosave && record.changed_for_autosave?)

            if association.updated?
              primary_key = Array(compute_primary_key(reflection, record)).map(&:to_s)
              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 = record._read_attribute(primary_key)
                self[foreign_key] = association_id unless self[foreign_key] == association_id
              end
              association.loaded!
            end

            saved if autosave
          end
        end
      end