replace_on_target

# File activerecord/lib/active_record/associations/collection_association.rb, line 417
      def replace_on_target(record, index, skip_callbacks)
        callback(:before_add, record) unless skip_callbacks
        yield(record) if block_given?

        if index
          @target[index] = record
        else
          @target << record
        end

        callback(:after_add, record) unless skip_callbacks
        set_inverse_instance(record)

        record
      end