atomic_first_or_create

AtomicFirstOrCreate

ActiveRecord first_or_create alternative that retries on RecordNotUnique exception.

first_or_create does not guarantee uniqueness by itself, and if there is a uniqueness constraint on the database, it may fail with a RecordNotUnique exception. This gem adds atomic_first_or_create to ActiveRecord::Relation, which, in conjunction with a uniqueness constraint, provides the correct behaviour.

Installation

Add this line to your application's Gemfile:

gem 'atomic_first_or_create'

And then execute:

$ bundle

Or install it yourself as:

$ gem install atomic_first_or_create

Solution

As documented by the rails team, methods like first_or_create or find_or_create are not atomic:

"Please note this method is not atomic, it runs first a SELECT, and if there are no results an INSERT is attempted. If there are other threads or processes there is a race condition between both calls and it could be the case that you end up with two similar records."

The documentation suggests using a UNIQUE constraint on the rows which you wish to enforce uniqueness, and retrying when the query raises the exception ActiveRecord::RecordNotUnique. This gem provides a method atomic_first_or_create which does exactly that.

Usage

Finds the first record with the given attributes, or creates a record with the attributes if one is not found:

# Find the first user with uid "abc123" or create a new one.
User.where(uid: 'abc123').atomic_first_or_create(name: 'John', uid: 'abc123')
# => #<User id: 1, uid: "abc123", name: 'John'>

# Find the first user with uid "abc123" or create a new one.
# We already have one so the existing record will be returned.
User.where(uid: 'abc123').atomic_first_or_create(name: 'John', uid: 'abc123')
# => #<User id: 1, uid: "abc123", name: 'John'>

# Find the first user with uid "abc1234" or create a new one
User.where(uid: 'abc123').atomic_first_or_create(name: 'Johansson', uid: 'abc1234')
# => #<User id: 2, uid: "abc1234", name: 'Johansson'>