Question

Activerecord question.. How to optimize this query..

Prefectures (have many) Cities (have many) Shops (have many) Sales (have many) Brands

I’d like to get a list of one sale per prefecture, which is yet to end.. and then list the brands available at the sale.

The nesting is making this kind of tricky for me!

Here is what I came up with, though it’s pretty ugly & I think it can be optimized at the query level rather than getting all unfinished sales..

#Get all sales which are yet to finish, ordered by finish date
upcoming_sales = Sale.includes([{:shop => {:city => :prefecture}}, :brands])
  .where("finish > ?", Date.today)
  .order('start ASC, finish ASC')
  .select(['shop.city.prefecture.name', 'brands.name'])

#filter down to a single sale per prefecture
 @sales = upcoming_sales.each_with_object({}){ |s,o|
    o[s.shop.city.prefecture.name] = o[s.shop.city.prefecture.name] ||= s
  }     

Answer 1

How about something like this?

class Sale < ActiveRecord::Base

    belongs_to :shop
    has_many :brands

    def self.first_sale_per_prefecture()
        first_sale_id_per_prefecture = %(
            select max(sales.id) 
            from sales
            inner join shops on shop_id = shops.id
            inner join cities on city_id = cities.id
            where finish > #{Date.today}
            group by prefecture_id
            order by finish desc)

        where("sales.id in (#{first_sale_id_per_prefecture})").includes(:brands, :shop => {:city => :prefecture})
    end
end