replace_on_target

# File activerecord/lib/active_record/associations/collection_association.rb, line 440
        def replace_on_target(record, index, skip_callbacks)
          callback(:before_add, record) unless skip_callbacks

          set_inverse_instance(record)

          @_was_loaded = true

          yield(record) if block_given?

          if index
            target[index] = record
          elsif @_was_loaded || !loaded?
            target << record
          end

          callback(:after_add, record) unless skip_callbacks

          record
        ensure
          @_was_loaded = nil
        end