build_count_subquery

# File activerecord/lib/active_record/relation/calculations.rb, line 403
      def build_count_subquery(relation, column_name, distinct)
        if column_name == :all
          relation.select_values = [ Arel.sql(FinderMethods::ONE_AS_ONE) ] unless distinct
        else
          column_alias = Arel.sql("count_column")
          relation.select_values = [ aggregate_column(column_name).as(column_alias) ]
        end

        subquery = relation.arel.as(Arel.sql("subquery_for_count"))
        select_value = operation_over_aggregate_column(column_alias || Arel.star, "count", false)

        Arel::SelectManager.new(subquery).project(select_value)
      end