replace

# File activerecord/lib/active_record/associations/has_one_association.rb, line 42
        def replace(record, save = true)
          raise_on_type_mismatch!(record) if record

          return target unless load_target || record

          assigning_another_record = target != record
          if assigning_another_record || record.has_changes_to_save?
            save &&= owner.persisted?

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

              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