replace

# File activerecord/lib/active_record/associations/has_one_association.rb, line 24
      def replace(record, save = true)
        raise_on_type_mismatch!(record) if record
        load_target

        return self.target if !(target || record)
        if (target != record) || record.changed?
          save &&= owner.persisted?

          transaction_if(save) do
            remove_target!(options[:dependent]) if target && !target.destroyed?

            if record
              set_owner_attributes(record)
              set_inverse_instance(record)

              if save && !record.save
                nullify_owner_attributes(record)
                set_owner_attributes(target) if target
                raise RecordNotSaved, "Failed to save the new associated #{reflection.name}."
              end
            end
          end
        end

        self.target = record
      end