execute_grouped_calculation

# File activerecord/lib/active_record/relation/calculations.rb, line 308
      def execute_grouped_calculation(operation, column_name, distinct) #:nodoc:
        group_fields = group_values

        if group_fields.size == 1 && group_fields.first.respond_to?(:to_sym)
          association  = klass._reflect_on_association(group_fields.first)
          associated   = association && association.belongs_to? # only count belongs_to associations
          group_fields = Array(association.foreign_key) if associated
        end
        group_fields = arel_columns(group_fields)

        group_aliases = group_fields.map { |field|
          field = connection.visitor.compile(field) if Arel.arel_node?(field)
          column_alias_for(field.to_s.downcase)
        }
        group_columns = group_aliases.zip(group_fields)

        aggregate_alias = column_alias_for("#{operation} #{column_name.to_s.downcase}")

        select_values = [
          operation_over_aggregate_column(
            aggregate_column(column_name),
            operation,
            distinct).as(aggregate_alias)
        ]
        select_values += self.select_values unless having_clause.empty?

        select_values.concat group_columns.map { |aliaz, field|
          if field.respond_to?(:as)
            field.as(aliaz)
          else
            "#{field} AS #{aliaz}"
          end
        }

        relation = except(:group).distinct!(false)
        relation.group_values  = group_fields
        relation.select_values = select_values

        calculated_data = skip_query_cache_if_necessary { @klass.connection.select_all(relation.arel, nil) }

        if association
          key_ids     = calculated_data.collect { |row| row[group_aliases.first] }
          key_records = association.klass.base_class.where(association.klass.base_class.primary_key => key_ids)
          key_records = Hash[key_records.map { |r| [r.id, r] }]
        end

        Hash[calculated_data.map do |row|
          key = group_columns.map { |aliaz, col_name|
            type = type_for(col_name) do
              calculated_data.column_types.fetch(aliaz, Type.default_value)
            end
            type_cast_calculated_value(row[aliaz], type)
          }
          key = key.first if key.size == 1
          key = key_records[key] if associated

          type = calculated_data.column_types.fetch(aggregate_alias) { type_for(column_name) }
          [key, type_cast_calculated_value(row[aggregate_alias], type, operation)]
        end]
      end