Commit 9c1a3bce authored by Yorick Peterse's avatar Yorick Peterse

Merge branch 'ab-docs-find-or-create' into 'master'

Document pattern for .find_or_create and siblings

See merge request gitlab-org/gitlab-ce!20649
parents bf4ce93f 4dac4bfc
......@@ -243,3 +243,45 @@ WHERE EXISTS (
```
[gin-index]: http://www.postgresql.org/docs/current/static/gin.html
## `.find_or_create_by` is not atomic
The inherent pattern with methods like `.find_or_create_by` and
`.first_or_create` and others is that they are not atomic. This means,
it first runs a `SELECT`, and if there are no results an `INSERT` is
performed. With concurrent processes in mind, there is a race condition
which may lead to trying to insert two similar records. This may not be
desired, or may cause one of the queries to fail due to a constraint
violation, for example.
Using transactions does not solve this problem.
The following pattern should be used to avoid the problem:
```ruby
Project.transaction do
begin
User.find_or_create_by(username: "foo")
rescue ActiveRecord::RecordNotUnique
retry
end
end
```
If the above block is run inside a transaction and hits the race
condition, the transaction is aborted and we cannot simply retry (any
further queries inside the aborted transaction are going to fail). We
can employ [nested transactions](http://api.rubyonrails.org/classes/ActiveRecord/Transactions/ClassMethods.html#module-ActiveRecord::Transactions::ClassMethods-label-Nested+transactions)
here to only rollback the "inner transaction". Note that `requires_new: true` is required here.
```ruby
Project.transaction do
begin
User.transaction(requires_new: true) do
User.find_or_create_by(username: "foo")
end
rescue ActiveRecord::RecordNotUnique
retry
end
end
```
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