models_concerns_has_one_auto_created.rb

module Concerns
  module HasOneAutoCreated
    extend ActiveSupport::Concern

    module ClassMethods
      def has_one_auto_created(association_name, options = {})
        has_one association_name, options

        alias_method "existing_#{association_name}", association_name

        # Returns the existing associated record, or a newly-created one.
        define_method(association_name) do
          transaction do
            send("existing_#{association_name}") || send("create_#{association_name}")
          end
        end

        # Yields with an existing or new association record, saving the record after the block completes.
        # All of this happens inside of a single database transaction.
        define_method("updating_#{association_name}") do |&block|
          transaction do
            record = send("existing_#{association_name}") || send("build_#{association_name}")
            block.call(record)
            record.save!
          end
        end
      end
    end
  end
end

models_gadget.rb

class Gadget < ActiveRecord::Base
  include Concerns::HasOneAutoCreated

  has_one_auto_created :gadget_status
end

models_gadget_spec.rb

require 'rails_helper'

describe Gadget, type: :model do
  describe '#gadget_part' do
    subject { FactoryGirl.create(:gadget) }

    context 'with an existing record' do
      let!(:gadget_part) { FactoryGirl.create(:gadget_part, gadget: subject) }

      it 'returns the existing record' do
        expect(subject.gadget_part).to eq(gadget_part)
      end
    end
    context 'without existing record' do
      it 'creates a new record' do
        expect(subject.gadget_part).to be_kind_of(GadgetPart)
        expect(subject.gadget_part).to be_persisted
        expect(subject.gadget_part).to have_attributes(gadget_id: subject.id)
      end
    end
  end

  describe '#updating_gadget_part' do
    subject { FactoryGirl.create(:gadget) }

    context 'with an existing record' do
      let!(:gadget_part) { FactoryGirl.create(:gadget_part, gadget: subject) }

      it 'yields the existing record' do
        expect { |b| subject.updating_gadget_part(&b) }
          .to yield_with_args(gadget_part)
      end
      it 'saves the new record after yielding it' do
        subject.updating_gadget_part do |gadget_part|
          expect(gadget_part).to receive(:save!)
        end
      end
    end
    context 'without existing record' do
      it 'yields a new record' do
        fake_gadget_part = double('gadget_part', save!: true)
        allow(subject).to receive(:build_gadget_part).and_return(fake_gadget_part)
        expect { |b| subject.updating_gadget_part(&b) }
          .to yield_with_args(fake_gadget_part)
      end
      it 'creates a correctly-formed GadgetPart' do
        subject.updating_gadget_part do |gadget_part|
          expect(gadget_part).to be_kind_of(GadgetPart)
          expect(gadget_part).to_not be_persisted
          expect(gadget_part).to have_attributes(gadget_id: subject.id)
        end
      end
      it 'saves the new record after yielding it' do
        subject.updating_gadget_part do |gadget_part|
          expect(gadget_part).to receive(:save!)
        end
      end
    end
  end
end

models_gadget_status.rb

class GadgetPart < ActiveRecord::Base
  belongs_to :gadget
end