Single Association Replace Behavior

arm.rb

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

bandit.rb

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

bandit_test.rb

require 'test_helper'

class BanditTest < ActiveSupport::TestCase
  def setup
    @bandit = Bandit.create
  end

  test 'grow an arm' do
    assert_instance_of Arm, @bandit.create_arm!
  end

  test 'try to replace an arm' do
    arm = @bandit.create_arm!
    assert arm.persisted?
    assert_raises(ActiveRecord::RecordInvalid) { @bandit.create_arm! }
    # now the old arm is gone
    @bandit.reload
    assert_instance_of NilClass, @bandit.arm
    assert_raises(ActiveRecord::RecordNotFound) { Arm.find(arm.id) }
  end
end

create_arms.rb

class CreateArms < ActiveRecord::Migration
  def change
    create_table :arms do |t|
      t.references :bandit
      t.timestamps null: false
    end
  end
end