replace

# File activerecord/lib/active_record/associations/association_collection.rb, line 353
      def replace(other_array)
        other_array.each { |val| raise_on_type_mismatch(val) }

        load_target
        other   = other_array.size < 100 ? other_array : other_array.to_set
        current = @target.size < 100 ? @target : @target.to_set

        transaction do
          delete(@target.select { |v| !other.include?(v) })
          concat(other_array.select { |v| !current.include?(v) })
        end
      end