slug.rb

module Slug

  def generate_slug
    temp_slug = SecureRandom.hex 8
    # ensure slug is unique
    while self.class.where(slug: temp_slug).count > 0
      temp_slug = SecureRandom.hex 8
    end
    self.slug = temp_slug
  end

end

slug_spec_convoluted.rb

require 'spec_helper'

class Abc < OpenStruct
  include Slug
  @@instances = []
  def initialize(attributes = {})
    super(attributes)
    @@instances << self
  end
  def self.where(attributes)
    @@instances.find_all do |instance|
      attributes.all? do |key, value|
        instance.send(key) == value
      end
    end
  end
end

describe Slug do
  let(:object) { Abc.new }
  let(:hex_regex) { /\A[\d|abcdef]{16}\z/ }

  before do
    #object.stub_chain(:class, :where, :count).and_return(
  end

  describe 'generate_slug' do
    context 'normal behaviour' do
      before do
        SecureRandom.should_receive(:hex).once.and_return('0123456789abcdef')
      end
      specify do
        expect(object.generate_slug).to match hex_regex
      end
    end

    context 'duplicate slug' do
      let!(:other_object) { Abc.new(slug: '153d0849b8c7ac74') }
      before do
        # second value of and_return will be returned on second call of .hex 
        SecureRandom.should_receive(:hex).twice.and_return other_object.slug, '0123456789abcdef'
        object.generate_slug
      end

      specify do
        expect(object.slug).to_not eql other_object.slug
        expect(object.slug).to match hex_regex
      end
    end
  end

end

slug_spec_short.rb

require 'spec_helper'

describe Slug do
  let(:klass) { Class.new { include Slug; attr_accessor :slug } }
  let(:object) { klass.new }
  let(:hex_regex) { /\A[\d|abcdef]{16}\z/ }

  describe 'generate_slug' do
    context 'normal behaviour' do
      before do
        klass.stub_chain(:where, :count).and_return 0
        SecureRandom.should_receive(:hex).once.and_return('0123456789abcdef')
        object.generate_slug
      end
      specify do
        expect(object.slug).to match hex_regex
      end
    end

    context 'duplicate slug' do
      before do
        klass.stub_chain(:where, :count).and_return 1, 0
        # second value of and_return will be returned on second call of .hex 
        SecureRandom.should_receive(:hex).twice.and_return '111222333444555a', '0123456789abcdef'
        object.generate_slug
      end

      specify do
        expect(object.slug).to_not eql '111222333444555a'
        expect(object.slug).to match hex_regex
      end
    end
  end

end