replace

# File activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb, line 5
      def replace(record)
        if record.nil?
          @target = @owner[@reflection.primary_key_name] = @owner[@reflection.options[:foreign_type]] = nil
        else
          @target = (AssociationProxy === record ? record.target : record)

          @owner[@reflection.primary_key_name] = record_id(record)
          @owner[@reflection.options[:foreign_type]] = record.class.base_class.name.to_s

          @updated = true
        end

        set_inverse_instance(record, @owner)
        loaded
        record
      end