replace

# File activerecord/lib/active_record/associations/has_one_association.rb, line 7
      def replace(record, save = true)
        raise_on_type_mismatch(record) if record
        load_target

        # If target and record are nil, or target is equal to record,
        # we don't need to have transaction.
        if (target || record) && target != record
          transaction_if(save) do
            remove_target!(options[:dependent]) if target && !target.destroyed?
  
            if record
              set_owner_attributes(record)
              set_inverse_instance(record)
  
              if owner.persisted? && 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