https://github.com/rails/rails/pull/11001

rails-11001.rb

gem 'rails', '3.2.13'
require 'active_record'

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

ActiveRecord::Base.establish_connection(
  adapter: 'sqlite3',
  database: ':memory:'
)

ActiveRecord::Schema.define do
  create_table :firms, force: true do |t|
  end
  create_table :clients, force: true do |t|
    t.integer :firm_id
    t.string :description
  end
end

class Firm < ActiveRecord::Base
  has_many :clients
end

class Client < ActiveRecord::Base
  belongs_to :firm
end

firm = Firm.new
firm.clients << Client.create!
firm.save!

puts "#{firm.clients.count} clients. 1 expected."

updated_clients = firm.clients.update_all(description: 'Great!')
puts "#{updated_clients} clients updated. 1 expected."