has_one_association.rb

module ActiveRecord
  # = Active Record Belongs To Has One Association
  module Associations
    class HasOneAssociation < SingularAssociation #:nodoc:

      def handle_dependency
        case options[:dependent]
        when :restrict_with_exception
          raise ActiveRecord::DeleteRestrictionError.new(reflection.name) if load_target

        when :restrict_with_error
          if load_target
            record = klass.human_attribute_name(reflection.name).downcase
            owner.errors.add(:base, :"restrict_dependent_destroy.one", record: record)
            false
          end

        else
          delete
        end
      end

      def replace(record, save = true)
        raise_on_type_mismatch!(record) if record
        load_target

        return self.target if !(target || record)

        assigning_another_record = target != record
        if assigning_another_record || record.changed?
          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

      def delete(method = options[:dependent])
        if load_target
          case method
            when :delete
              target.delete
            when :destroy
              target.destroy
            when :nullify
              target.update_columns(reflection.foreign_key => nil)
          end
        end
      end

      private

        # The reason that the save param for replace is false, if for create (not just build),
        # is because the setting of the foreign keys is actually handled by the scoping when
        # the record is instantiated, and so they are set straight away and do not need to be
        # updated within replace.
        def set_new_record(record)
          replace(record, false)
        end

        def remove_target!(method)
          case method
            when :delete
              target.delete
            when :destroy
              target.destroy
            else
              nullify_owner_attributes(target)

              if target.persisted? && owner.persisted? && !target.save
                set_owner_attributes(target)
                raise RecordNotSaved, "Failed to remove the existing associated #{reflection.name}. " +
                                      "The record failed to save after its foreign key was set to nil."
              end
          end
        end

        def nullify_owner_attributes(record)
          record[reflection.foreign_key] = nil
        end

        def transaction_if(value)
          if value
            reflection.klass.transaction { yield }
          else
            yield
          end
        end
    end
  end
end