Optimize ActiveRecord query

Question

I have 2 tables in database: cars and domains. One car can have many domains and one domain can have many cars.

In my project three models:

class Car < ActiveRecord::Base

  has_many :cars_domains
  has_many :domains, :through => :cars_domains

...

class Domain < ActiveRecord::Base

    has_many :cars_domains
    has_many :cars, :through => :cars_domains
...

class CarsDomain < ActiveRecord::Base
  belongs_to :car
  belongs_to :domain
end

I want to see cars which without domain:

@cars = Car.find(:all, :conditions => ['id not in(select car_id from cars_domains where domain_id = ?)', params[:domain_id]])

It's work, but I think it's very difficult. Maybe possible to do it more simple?

I'd use a left join rather than a subquery

Frederick Cheung
– Frederick Cheung

2012-08-29 13:01:09 +00:00
Commented Aug 29, 2012 at 13:01 — Frederick Cheung
– Frederick Cheung, Commented Aug 29, 2012 at 13:01

agmcleod · Accepted Answer · 2012-08-29 14:35:01Z

1

Tried this in a quick app I created:

domain = Domain.find(params[:domain_id])
Car.includes(:cars_domain).where('cars_domain.domain_id <> ?', domain.id)

The reason I query the domain object is because I'm always weary about passing a value from the request headers as a query parameter in SQL.

answered Aug 29, 2012 at 14:35

agmcleod

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