Reproducing Test for replacing has_one relation with uniqueness constraint

has_one_relation_test.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'rack', github: 'rack/rack'
    gem 'i18n', github: 'svenfuchs/i18n'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'rails'
require 'active_record'

require 'minitest/autorun'
require 'rack/test'

class Arm < ActiveRecord::Base
  belongs_to :bandit, inverse_of: :arm

  # instead of validating this here, you could also add a uniqueness
  # constraint through a migration. Behavior would be similiar.
  validates :bandit, uniqueness: true
end

class Bandit < ActiveRecord::Base
  has_one :arm, dependent: :destroy, inverse_of: :bandit
end

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
migration = ActiveRecord::Migration.new
migration.create_table :bandits do |table|
  table.timestamps null: false
end

migration.create_table :arms do |table|
  table.references :bandit
  table.timestamps null: false
end

class ReplacingHasOneRelationWithUniquenessConstraintTest < Minitest::Test
  include Rack::Test::Methods

  def setup
    @bandit = Bandit.create!
    @bandit.create_arm!
  end

  def test_create_arm_should_replace_arm
    @bandit.create_arm!
  end
end