validate_has_one_association

# File activerecord/lib/active_record/autosave_association.rb, line 329
      def validate_has_one_association(reflection)
        association = association_instance_get(reflection.name)
        record      = association && association.reader
        return unless record && (record.changed_for_autosave? || custom_validation_context?)

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

        association_valid?(association, record)
      end