-[ ] [API support added](https://docs.gitlab.com/ee/development/api_styleguide.html)
-[ ] [Tests added for this feature/bug](https://docs.gitlab.com/ee/development/testing_guide/index.html)
- Conforms to the [code review guidelines](https://docs.gitlab.com/ee/development/code_review.html)
-[ ] Has been reviewed by a Backend [maintainer](https://about.gitlab.com/handbook/engineering/#maintainer)
-[ ] Has been reviewed by a Database [specialist](https://about.gitlab.com/team/structure/#specialist)
-[ ] Conforms to the [merge request performance guidelines](https://docs.gitlab.com/ee/development/merge_request_performance_guidelines.html)
-[ ] Conforms to the [style guides](https://gitlab.com/gitlab-org/gitlab-ee/blob/master/CONTRIBUTING.md#style-guides)
-[ ] If you have multiple commits, please combine them into a few logically organized commits by [squashing them](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
- [ ] Internationalization required/considered
- [ ] If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
- [ ] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
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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.transactiondo
begin
User.find_or_create_by(username: "foo")
rescueActiveRecord::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.
# Import multiple repositories by uploading a manifest file
GitLab allows you to import all the required git repositories
based a manifest file like the one used by the Android repository.
based a manifest file like the one used by the [Android repository](https://android.googlesource.com/platform/manifest/+/2d6f081a3b05d8ef7a2b1b52b0d536b2b74feab4/default.xml).
This feature can be very handy when you need to import a project with many repositories like Android Open Source Project (AOSP).
