find_unique_index_for

# File activerecord/lib/active_record/insert_all.rb, line 59
      def find_unique_index_for(unique_by)
        match = Array(unique_by).map(&:to_s)

        if index = unique_indexes.find { |i| match.include?(i.name) || i.columns == match }
          index
        else
          raise ArgumentError, "No unique index found for #{unique_by}"
        end
      end