rails scope to check if association does NOT exist

associations, ruby-on-rails, scopes, sql

Solution

Rails 4 makes this too easy :)

Foo.where.not(id: Bar.select(:foo_id).uniq)

this outputs the same query as jdoe's answer

SELECT "foos".* 
FROM "foos" 
WHERE "foos"."id" NOT IN (
  SELECT DISTINCT "bars"."foo_id"
  FROM "bars" 
)

And as a scope:

scope :lonely, -> { where.not(id: Bar.select(:item_id).uniq) }

Problem

I am looking toward writing a scope that returns all records that do not have a particular association. foo.rb ``` class Foo < ActiveRecord::Base has_many :bars end ``` bar.rb ``` class Bar < ActiveRecord::Base belongs_to :foo end ``` I want a scope that can find all of the `Foo's` that dont have any `bars`. It's easy to find the ones that have an association using `joins`, but I haven't found a way to do the opposite.

Original source