Handling Uniqueness and Duplicates in Rails

The uniqueness validation provided by Rails is not enough to ensure you don't get duplicates. Adding indices causes 500s. This presentation can help!

Transcript

2. gmalette @ ! "
3. do not duplicate This talk is called “Do not duplicate” We’ll explore the pitfalls to avoid when dealing with uniqueness constraints in Rails
4. validates :email, uniqueness: true Show of hands, who has code that looks like this?
5. It works as expected when you try to use the same email as someone else
6. how?
7. [Rails] User.create(email: "gmalette@gmail.com") [SQL] SELECT 1 AS one FROM `users` WHERE (`users`.`email` = 'gmalette@gmail.com') LIMIT 1 [SQL] #=> 0 [Rails] [Rails] INSERT INTO `users` (`email`) VALUES ('gmalette@gmail.com') Rails performs a SELECT to see that no other record fit the uniqueness constraint
8. race conditions Let’s talk about race conditions
9. [Rails] User.create(email: “gmalette@gmail.com”) [SQL] SELECT 1 AS one FROM `users` WHERE (`users`.`email` = 'gmalette@gmail.com') LIMIT 1 [SQL] #=> 0 [Rails] [Rails] INSERT INTO `users` (`email`) VALUES ('gmalette@gmail.com') Worker 1 [Rails] User.create(email: "gmalette@gmail.com") [SQL] SELECT 1 AS one FROM `users` WHERE (`users`.`email` = 'gmalette@gmail.com') LIMIT 1 [SQL] #=> 0 [Rails] [Rails] INSERT INTO `users` (`email`) VALUES ('gmalette@gmail.com') Worker 2 [Rails] [Rails] Two simultaneous requests trying to create the same user
10. You get unexpected results!
11. change_table :users do |m| m.index :email, unique: true end To fix this we’ll add a uniqueness constraint in the database
12. [Rails] User.create(email: “gmalette@gmail.com”) [SQL] SELECT 1 AS one FROM `users` WHERE (`users`.`email` = 'gmalette@gmail.com') LIMIT 1 [SQL] #=> 0 [Rails] [Rails] INSERT INTO `users` (`email`) VALUES ('gmalette@gmail.com') Worker 1 [Rails] User.create(email: "gmalette@gmail.com") [SQL] SELECT 1 AS one FROM `users` WHERE (`users`.`email` = 'gmalette@gmail.com') LIMIT 1 [SQL] #=> 0 [Rails] [Rails] INSERT INTO `users` (`email`) VALUES ('gmalette@gmail.com') Worker 2 [Rails]
13. ActiveRecord::RecordNotUnique MySQL raises an error Rails doesn’t know how to handle it
15. rescue_from ActiveRecord::RecordNotUnique do # Do something sane end The first thing one can do is add this to the controller I don’t like `rescue_from`
16. This gem can help us
17. gem 'activerecord-rescue_from_duplicate'
18. validates :email, uniqueness: { rescue_from_duplicate: true } Simply add `rescue_from_duplicate: true`
19. [Rails] User.create(email: “gmalette@gmail.com”) [SQL] SELECT 1 AS one FROM `users` WHERE (`users`.`email` = 'gmalette@gmail.com') LIMIT 1 [SQL] #=> 0 [Rails] [Rails] INSERT INTO `users` (`email`) VALUES ('gmalette@gmail.com') Worker 1 [Rails] User.create(email: "gmalette@gmail.com") [SQL] SELECT 1 AS one FROM `users` WHERE (`users`.`email` = 'gmalette@gmail.com') LIMIT 1 [SQL] #=> 0 [Rails] [Rails] INSERT INTO `users` (`email`) VALUES ('gmalette@gmail.com') Worker 2 [Rails] [Rails]
20. user.save => false user.errors => #<ActiveModel::Errors @messages={:email=>["has already been taken"]}> When the save fails, it will behave as a validation error.
21. config.eager_load = true RescueFromDuplicate.missing_unique_indexes It comes with a few other utilities, like helping you find missing unique indexes in your app
22. caveat / gotchas
23. after_validation before_save before_create activerecord callbacks happen don’t put anything destructive in there
24. user.save! # => ActiveRecord::RecordNotSaved activerecord callbacks happen don’t put anything destructive in there
25. questions?
26. thanks!