test_update_all.rb

require 'active_record'
require 'logger'
require 'pp'

puts "Active Record #{ActiveRecord::VERSION::STRING}"

ActiveRecord::Base.establish_connection(
  :adapter  => 'mysql',
  :host     => '127.0.0.1',
  :user     => 'root',
  :database => 'test'
)

ActiveRecord::Schema.define do
  create_table :comments, :force => true do |t|
    t.integer :post_id
  end

  create_table :posts, :force => true do |t|
  end
end

class Post < ActiveRecord::Base
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

ActiveRecord::Base.logger = Logger.new(STDOUT)

Comment.joins(:post).where('posts.id IS NOT NULL').update_all({:post_id => 1}, nil, {:limit => 1, :order => "posts.id"})
# This should produce something like the following in the log output:
# UPDATE comments SET "post_id" = 1 WHERE (comments.id IN (SELECT id from (SELECT "comments".* FROM "comments" INNER JOIN "posts" ON "posts"."id" = "comments"."post_id" WHERE (posts.id IS NOT NULL) ORDER BY posts.id LIMIT 1) x))