replace

# File activerecord/lib/active_record/associations/has_one_association.rb, line 31
      def replace(obj, dont_save = false)
        load_target

        unless @target.nil? || @target == obj
          if dependent? && !dont_save
            case @reflection.options[:dependent]
            when :delete
              @target.delete unless @target.new_record?
              @owner.clear_association_cache
            when :destroy
              @target.destroy unless @target.new_record?
              @owner.clear_association_cache
            when :nullify
              @target[@reflection.primary_key_name] = nil
              @target.save unless @owner.new_record? || @target.new_record?
            end
          else
            @target[@reflection.primary_key_name] = nil
            @target.save unless @owner.new_record? || @target.new_record?
          end
        end

        if obj.nil?
          @target = nil
        else
          raise_on_type_mismatch(obj)
          set_belongs_to_association_for(obj)
          @target = (AssociationProxy === obj ? obj.target : obj)
        end

        set_inverse_instance(obj, @owner)
        @loaded = true

        unless @owner.new_record? or obj.nil? or dont_save
          return (obj.save ? self : false)
        else
          return (obj.nil? ? nil : self)
        end
      end