こんな libraries を書いたとする

libraries/awesome_mixin.rb

module Awesome
  module Mixin
    def foo
      node['foo']
    end
  end
end

Chef::Recipe.send(:include, Awesome::Mixin)
Chef::Resource.send(:include, Awesome::Mixin)

node なんかを使っていなければchefspecでlibraries内のメソッドをテストするのやり方で十分なのだが、node を使っている場合は次のようにする

spec/spec_helper.rb

require 'chefspec'

def cookbook_root
  File.expand_path("../..", __FILE__)
end

def cookbook_name
  'mycookbook'
end

spec/libraries/awesome_mixin_spec.rb

require_relative '../spec_helper'
require_relative '../../libraries/awesome_mixin'

describe Awesome::Mixin do
  let(:cookbook_collection) do
    cookbook_version = Chef::CookbookVersion.new(cookbook_name, cookbook_root)
    cookbook_versions = { cookbook_name => cookbook_version }
    Chef::CookbookCollection.new(cookbook_versions)
  end

  let(:events) do
    Chef::EventDispatch::Dispatcher.new
  end

  let(:node) do
    Chef::Node.new.tap {|node|
      node.set['foo'] = 'yay!'
    }
  end

  let(:run_context) do
    run_context = Chef::RunContext.new(node, cookbook_collection, events)
  end

  let(:recipe) do
    Chef::Recipe.new(cookbook_name, "test", run_context)
  end

  it do
    expect(recipe.foo).to eql('yay!')
  end
end

長い！！