limited_ids_for

# File activerecord/lib/active_record/relation/finder_methods.rb, line 413
      def limited_ids_for(relation)
        values = @klass.connection.columns_for_distinct(
          "#{quoted_table_name}.#{quoted_primary_key}", relation.order_values)

        relation = relation.except(:select).select(values).distinct!
        arel = relation.arel

        id_rows = @klass.connection.select_all(arel, "SQL", relation.bound_attributes)
        id_rows.map { |row| row[primary_key] }
      end