execute_grouped_calculation

# File activerecord/lib/active_record/relation/calculations.rb, line 328
      def execute_grouped_calculation(operation, column_name, distinct) # :nodoc:
        group_fields = group_values
        group_fields = group_fields.uniq if group_fields.size > 1

        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)

        column = aggregate_column(column_name)
        column_alias = column_alias_for("#{operation} #{column_name.to_s.downcase}")
        select_value = operation_over_aggregate_column(column, operation, distinct)
        select_value.as(column_alias)

        select_values = [select_value]
        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, "#{@klass.name} #{operation.capitalize}") }

        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 = key_records.index_by(&:id)
        end

        key_types = group_columns.each_with_object({}) do |(aliaz, col_name), types|
          types[aliaz] = type_for(col_name) do
            calculated_data.column_types.fetch(aliaz, Type.default_value)
          end
        end

        hash_rows = calculated_data.cast_values(key_types).map! do |row|
          calculated_data.columns.each_with_object({}).with_index do |(col_name, hash), i|
            hash[col_name] = row[i]
          end
        end

        if operation != "count"
          type = column.try(:type_caster) ||
            lookup_cast_type_from_join_dependencies(column_name.to_s) || Type.default_value
          type = type.subtype if Enum::EnumType === type
        end

        hash_rows.each_with_object({}) do |row, result|
          key = group_aliases.map { |aliaz| row[aliaz] }
          key = key.first if key.size == 1
          key = key_records[key] if associated

          result[key] = type_cast_calculated_value(row[column_alias], operation, type)
        end
      end