ActiveModel::Serializers

discount_schedule_serializer.rb

class Api::V1::DiscountScheduleSerializer < ActiveModel::Serializer
  attributes :id, ... ,:created_at, :updated_at, :number_of_reservations
  
  def created_at
    object.created_at.in_time_zone.iso8601 if object.created_at
  end

  def updated_at
    object.updated_at.in_time_zone.iso8601 if object.updated_at
  end

end

some_controller.rb

def some_action

 @objects = DiscountSchedule.joins(:day_of_week)
			    .includes(:some_other_model_1, :some_other_model_2)
                            .joins("LEFT JOIN LATERAL ( 
                                    SELECT 
                                      COUNT(1) number_of_reservations
                                    FROM reservations
                                    WHERE ... some conditions ...
                                    GROUP BY id
                                    LIMIT 1

                                  ) today_reservations_count ON true "
                              )
                              .where(is_active: true)
                              .select('number_of_reservations as number_of_reservations')
		
 render(
  json: ActiveModel::Serializer::CollectionSerializer.new(
  	@discount_schedules,
    serializer: Api::V1::DiscountScheduleSerializer
  ),
  status: :ok
 )	
	
end

xquery.sql

SELECT
  number_of_reservations as number_of_reservations,  ... very big list of model fields ...
FROM "discount_schedules" 
	INNER JOIN "day_of_week" ON "day_of_week"."id" = "discount_schedules"."id_day_of_week"  
        LEFT OUTER JOIN "some_other_model_1" ...
        LEFT OUTER JOIN "some_other_model_2" ...
	LEFT JOIN LATERAL (

				    SELECT 
                                    	COUNT(1) number_of_reservations
                                    FROM reservations
                                    WHERE ... some conditions ...
                                    GROUP BY id
                                    LIMIT 1

				) today_reservations_count ON true 
WHERE (discount_schedules.is_active = ('t'))