failure to detect optimistic locking collision on has_many :through relationship

Given the following models,

class User < ActiveRecord::Base

  has_many :courses

end

class Course < ActiveRecord::Base
  
  belongs_to :user

end

where Course has a lock version column defined,

create_table :courses do |t|
      ...
      t.integer :lock_version, :default => 0
      ...
    end

the following unit test to force a locking collision will pass,

test "optimistic_locking_behaviour" do
    first_course_instance = Course.find_by_name('Rails 101')
    second_course_instance = Course.find_by_name('Rails 101')
    
    first_course_instance.user = users(:one)
    second_course_instance.user = users(:two)
    
    assert first_course_instance.save, "First instance save succeeded"
    
    assert_raises ActiveRecord::StaleObjectError do
      second_course_instance.save
    end
  end

a look at log/test.log shows,

UPDATE `courses` SET `user_id` = 980190962, `updated_at` = '2011-01-26 13:02:53', `lock_version` = 1 WHERE (`courses`.`id` = 52995405 AND `courses`.`lock_version` = 0)

where the resulting query on the course object correctly checks the lock_version on update.

However, if the relationship between User and Course is changed to be many to many using has_many :through, there is no explicit checking of the lock_version on update,

class User < ActiveRecord::Base

  has_many :attendances
  has_many :registered_courses, :through => :attendances, :source => :course 

end

class Course < ActiveRecord::Base
  
  has_many :attendances
  has_many :attendees, :through => :attendances, :source => :user

end

class Attendance < ActiveRecord::Base
  
  belongs_to :user
  belongs_to :course, :counter_cache => true, :touch => true

end

and with a unit test to force a lock collision as before,

test "optimistic_locking_behaviour" do
    first_course_instance = Course.find_by_name('Rails 101')
    second_course_instance = Course.find_by_name('Rails 101')
    
    first_course_instance.touch
    assert first_course_instance.attendees << users(:one), "First instance succeeded"

    assert_raises ActiveRecord::StaleObjectError do
      second_course_instance.attendees << users(:two)
    end
  end

results in the following set of sql output,

SQL (0.0ms)  BEGIN
  SQL (0.4ms)  SHOW TABLES
  Course Load (0.3ms)  SELECT `courses`.* FROM `courses` WHERE (`courses`.`name` = 'Rails 101') LIMIT 1
  Course Load (0.1ms)  SELECT `courses`.* FROM `courses` WHERE (`courses`.`name` = 'Rails 101') LIMIT 1
  User Load (0.4ms)  SELECT `users`.* FROM `users` WHERE (`users`.`id` = 980190962) LIMIT 1
  SQL (0.1ms)  SAVEPOINT active_record_1
  SQL (2.8ms)  describe `attendances`
  AREL (0.2ms)  INSERT INTO `attendances` (`user_id`, `course_id`, `created_at`, `updated_at`) VALUES (980190962, 52995405, '2011-01-26 13:24:48', '2011-01-26 13:24:48')
  Course Load (0.3ms)  SELECT `courses`.* FROM `courses` WHERE (`courses`.`id` = 52995405) LIMIT 1
  AREL (0.2ms)  UPDATE `courses` SET `attendances_count` = COALESCE(`attendances_count`, 0) + 1, `lock_version` = COALESCE(`lock_version`, 0) + 1 WHERE (`courses`.`id` = 52995405)
  **AREL (0.1ms)  UPDATE `courses` SET `updated_at` = '2011-01-26 13:24:48' WHERE (`courses`.`id` = 52995405)**
  SQL (0.1ms)  RELEASE SAVEPOINT active_record_1
  User Load (0.1ms)  SELECT `users`.* FROM `users` WHERE (`users`.`id` = 298486374) LIMIT 1
  SQL (0.1ms)  SAVEPOINT active_record_1
  AREL (0.2ms)  INSERT INTO `attendances` (`user_id`, `course_id`, `created_at`, `updated_at`) VALUES (298486374, 52995405, '2011-01-26 13:24:48', '2011-01-26 13:24:48')
  Course Load (0.1ms)  SELECT `courses`.* FROM `courses` WHERE (`courses`.`id` = 52995405) LIMIT 1
  AREL (0.1ms)  UPDATE `courses` SET `attendances_count` = COALESCE(`attendances_count`, 0) + 1, `lock_version` = COALESCE(`lock_version`, 0) + 1 WHERE (`courses`.`id` = 52995405)
  AREL (0.1ms)  UPDATE `courses` SET `updated_at` = '2011-01-26 13:24:48' WHERE (`courses`.`id` = 52995405)
  SQL (0.1ms)  RELEASE SAVEPOINT active_record_1
  SQL (1.3ms)  ROLLBACK

Notice that the implicit update to the Course object (highlighted in bold), facilitated through the

:touch => true

option, doesn't take into account the lock_version of the row. Therefore, a locking collision will never be detected in the test. I think that either

a) ActiveRecord.touch should be aware of the lock_version column and handle this accordingly

or

b) the resulting update to the parent object (Course) should be check for a lock_version mismatch rather than simply increasing the lock version value by 1 i.e.

AREL (0.2ms)  UPDATE `courses` SET `attendances_count` = COALESCE(`attendances_count`, 0) + 1, `lock_version` = COALESCE(`lock_version`, 0) + 1 WHERE (`courses`.`id` = 52995405)