find_unique_index_for

# File activerecord/lib/active_record/insert_all.rb, line 71
      def find_unique_index_for(unique_by)
        return unique_by if !connection.supports_insert_conflict_target?

        name_or_columns = unique_by || model.primary_key
        match = Array(name_or_columns).map(&:to_s)

        if index = unique_indexes.find { |i| match.include?(i.name) || i.columns == match }
          index
        elsif match == primary_keys
          unique_by.nil? ? nil : ActiveRecord::ConnectionAdapters::IndexDefinition.new(model.table_name, "#{model.table_name}_primary_key", true, match)
        else
          raise ArgumentError, "No unique index found for #{name_or_columns}"
        end
      end