rails scope to check if association does NOT exist

I prefer to use squeel gem to build complex queries. It extends ActiveRecord with such a magic:

Foo.where{id.not_in Bar.select{foo_id}.uniq}

that builds the following query:

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

So,

# in Foo class
scope :lonely, where{id.not_in Bar.select{foo_id}.uniq}

is what you can use to build the requested scope.

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) }

in foo.rb

class Foo < ActiveRecord::Base    
  has_many :bars
  scope :lonely, lambda { joins('LEFT OUTER JOIN bars ON foos.id = bars.foo_id').where('bars.foo_id IS NULL') }
end

For Rails 5+ (Ruby 2.4.1 & Postgres 9.6)

I have 100 foos and 9900 bars. 99 of the foos each have 100 bars, and one of them has none.

Foo.left_outer_joins(:bars).where(bars: { foo_id: nil })

Produces one SQL query:

Foo Load (2.3ms)  SELECT  "foos".* FROM "foos" LEFT OUTER JOIN "bars" ON "bars"."foo_id" = "foos"."id" WHERE "bars"."foo_id" IS NULL

and returns the one Foo with no bars

The currently accepted answer Foo.where.not(id: Bar.select(:foo_id).uniq) is not working. It is producing two SQL queries:

Bar Load (8.4ms)  SELECT "bars"."foo_id" FROM "bars"
Foo Load (0.3ms)  SELECT  "foos".* FROM "foos" WHERE ("foos"."id" IS NOT NULL)

which returns all foos because all foos have an id that is not null.

It needs to be changed to Foo.where.not(id: Bar.pluck(:foo_id).uniq) to reduce it to one query and find our Foo, but it performs poorly in benchmarks

require 'benchmark/ips'
require_relative 'config/environment'

Benchmark.ips do |bm|
  bm.report('left_outer_joins') do
    Foo.left_outer_joins(:bars).where(bars: { foo_id: nil })
  end

  bm.report('where.not') do
    Foo.where.not(id: Bar.pluck(:foo_id).uniq)
  end

  bm.compare!
end

Warming up --------------------------------------
    left_outer_joins     1.143k i/100ms
           where.not     6.000  i/100ms
Calculating -------------------------------------
    left_outer_joins     13.659k (± 9.0%) i/s -     68.580k in   5.071807s
           where.not     70.856  (± 9.9%) i/s -    354.000  in   5.057443s

Comparison:
    left_outer_joins:    13659.3 i/s
           where.not:       70.9 i/s - 192.77x  slower