build_count_subquery

# File activerecord/lib/active_record/relation/calculations.rb, line 341
    def build_count_subquery(relation, column_name, distinct)
      column_alias = Arel.sql('count_column')
      subquery_alias = Arel.sql('subquery_for_count')

      aliased_column = aggregate_column(column_name == :all ? 1 : column_name).as(column_alias)
      relation.select_values = [aliased_column]
      subquery = relation.arel.as(subquery_alias)

      sm = Arel::SelectManager.new relation.engine
      select_value = operation_over_aggregate_column(column_alias, 'count', distinct)
      sm.project(select_value).from(subquery)
    end