replace_records

# File activerecord/lib/active_record/associations/collection_association.rb, line 561
        def replace_records(new_target, original_target)
          delete(target - new_target)

          unless concat(new_target - target)
            @target = original_target
            raise RecordNotSaved, "Failed to replace #{reflection.name} because one or more of the "                                    "new records could not be saved."
          end

          target
        end