连接多个表时，Rails / MySQL查询获取计数

Question

I have a rails app with the following structure: A Dealer can have many Cars, which in turn can have many different Promotions on it (car_promotions is a table linking the promotion to the car itself).

What I am trying to do is for each dealer, get a count of the number of each different car_promotion they currently have. So for example, if there is a promotion called "5% off" and one called "free shipping", it would fine the number of cars that have the first promotion, and the number of cars that have the second promotion.

Dealer.rb:

has_many :cars, :promotions

Car.rb:

has_many :car_promotions
attr_accessor :dealer_id

CarPromotion.rb:

belongs_to :car, :promotion.rb
attr_accessor :car_id, :promotion_id

Promotion.rb

belongs_to :dealer

Currently what I am doing is doing joining the cars and car_promotions models, and then for each promotion the dealer has in their database, cycle through them and getting the number of rows in my join which refers to the promotion. This is working, but its really slow, so I am wondering how I can optimize it (using either rails queries, or mysql queries). My sql query writing isn't great, so I am having some issues getting it to work correctly.

I also tried using a group_by on my joins, but I wasn't sure how to get that working, since if I just do a group_by directly on the joins, it doesn't have a reference to the promotion_id.

Any ideas? Thanks,

Answer 1

Once you go beyond two tables, I find often find it easier to write raw SQL. This might do what you want:

SELECT COUNT(cp.id), d.id
FROM car_promotians cp
  INNER JOIN cars c ON cp.car_id = c.id
  INNER JOIN dealers d ON c.dealer_id = d.id
GROUP BY d.id

Getting an efficient SQL query via ActiveRecord is probably possible. If you post your database schema, it might be easier for someone to help you.