Overkill but interesting learning experience for me about metaprogramming in Ruby.

acts_as_owned.rb

module ActsAsOwned
  def acts_as_owned(options = {})
    raise 'Must define owner lambda' unless options[:owner] && options[:owner].class == Proc
    define_method(:owner, &options[:owner])
  end
end
ActiveRecord::Base.send :extend, ActsAsOwned

acts_as_owned_test.rb

require 'test_helper'

describe 'ActsAsOwned' do
  before do
    class ActsAsOwned::Mocker
      extend ActsAsOwned
    end
  end

  it 'when owner not specified must throw' do
    #User.methods(true).any? { |m| m == :owner }.must_equal false
    #user = FactoryGirl.build(:user)
    mocker = ActsAsOwned::Mocker
    proc { mocker.acts_as_owned }.must_raise RuntimeError
  end

  it 'when owner not Proc must throw' do
    #User.methods(true).any? { |m| m == :owner }.must_equal false
    #user = FactoryGirl.build(:user)
    mocker = ActsAsOwned::Mocker
    proc { mocker.acts_as_owned }.must_raise RuntimeError
  end

  describe 'when the owner is specified' do
    before  do
      class ActsAsOwned::Mocker
        TEST_VALUE = SecureRandom.base64
        acts_as_owned owner: -> { TEST_VALUE }
      end
    end

    it 'owner must return value specified to acts_as_owned' do
      ActsAsOwned::Mocker.new.owner.must_equal ActsAsOwned::Mocker::TEST_VALUE
    end
  end
end