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failure to detect optimistic locking collision on has_many :through relationship
Reported by Cathal O' Riordan | January 26th, 2011 @ 01:42 PM
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)
Comments and changes to this ticket
-
rails April 27th, 2011 @ 01:00 AM
- State changed from new to open
This issue has been automatically marked as stale because it has not been commented on for at least three months.
The resources of the Rails core team are limited, and so we are asking for your help. If you can still reproduce this error on the 3-0-stable branch or on master, please reply with all of the information you have about it and add "[state:open]" to your comment. This will reopen the ticket for review. Likewise, if you feel that this is a very important feature for Rails to include, please reply with your explanation so we can consider it.
Thank you for all your contributions, and we hope you will understand this step to focus our efforts where they are most helpful.
-
rails April 27th, 2011 @ 01:00 AM
- State changed from open to stale
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