expand_attribute_names_for_aggregates

# File activerecord/lib/active_record/base.rb, line 1087
        def expand_attribute_names_for_aggregates(attribute_names)
          attribute_names.map { |attribute_name|
            unless (aggregation = reflect_on_aggregation(attribute_name.to_sym)).nil?
              aggregate_mapping(aggregation).map do |field_attr, _|
                field_attr.to_sym
              end
            else
              attribute_name.to_sym
            end
          }.flatten
        end