Rails 6 adds create_or_find_by and create_or_find_by!

Rails 6 added create_or_find_by and create_or_find_by!. Both of these methods rely on unique constraints on the database level. If creation fails, it is because of the unique constraints on one or all of the given columns, and it will try to find the record using find_by!.

create_or_find_by is an improvement over find_or_create_by because find_or_create_by first queries for the record, and then inserts it if none is found. This could lead to a race condition.

As mentioned by DHH in the pull request, create_or_find_by has a few cons too:

• The table must have unique constraints on the relevant columns.
• This method relies on exception handling, which is generally slower.

create_or_find_by! raises an exception when creation fails because of the validations.

Let's see how both methods work in Rails 6.0.0.beta2.

Rails 6.0.0.beta2

> > class CreateUsers < ActiveRecord::Migration[6.0]
> > def change
> > create_table :users do |t|
> > t.string :name, index: { unique: true }
> >
> >       t.timestamps
> >     end
> >
> > end
> > end

> > class User < ApplicationRecord
> > validates :name, presence: true
> > end

> > User.create_or_find_by(name: 'Amit')
> > BEGIN
> > INSERT INTO "users" ("name", "created_at", "updated_at") VALUES ($1, $2, \$3) RETURNING "id" [["name", "Amit"], ["created_at", "2019-03-07 09:33:23.391719"], ["updated_at", "2019-03-07 09:33:23.391719"]]
> > COMMIT

=> #<User id: 1, name: "Amit", created_at: "2019-03-07 09:33:23", updated_at: "2019-03-07 09:33:23">

> > User.create_or_find_by(name: 'Amit')
> > BEGIN
> > INSERT INTO "users" ("name", "created_at", "updated_at") VALUES ($1, $2, \$3) RETURNING "id" [["name", "Amit"], ["created_at", "2019-03-07 09:46:37.189068"], ["updated_at", "2019-03-07 09:46:37.189068"]]
> > ROLLBACK

=> #<User id: 1, name: "Amit", created_at: "2019-03-07 09:33:23", updated_at: "2019-03-07 09:33:23">

> > User.create_or_find_by(name: nil)
> > BEGIN
> > COMMIT

=> #<User id: nil, name: nil, created_at: nil, updated_at: nil>

> > User.create_or_find_by!(name: nil)

=> Traceback (most recent call last):
1: from (irb):2
ActiveRecord::RecordInvalid (Validation failed: Name can't be blank)

Here is the relevant pull request.

Also note, create_or_find_by can lead to primary keys running out, if the type of primary key is int. This happens because each time create_or_find_by hits ActiveRecord::RecordNotUnique, it does not rollback auto-increment of the primary key. The problem is discussed in this pull request.