This is a little helper method which take the path of a file relative to rails root directory and "require" that file in the test. Comes handy for the isolated tests in a rails project. Interestingly enough, I wrote this via TDD too!

some_domain_logic_spec.rb

#you can use this easily like the following in your spec files:

reuqire_relative "../spec_helper_lite"
reuqire_relative_to_root "lib/some_domain_logic"

describe "some domain logic" do
    #specs will be here then
end

spec_helper_lite.rb

def require_relative_to_root(file_path)
  rails_root = find_rails_root(File.dirname(__FILE__))
  require(File.expand_path("#{rails_root}/#{file_path}"))
end

def find_rails_root(dir)
  return dir if is_root?(dir) 
  find_rails_root(File.expand_path(dir + "/../"))
end

def is_root?(directory)
  entries = Dir.entries(directory)
  return entries.include?("app") && 
         entries.include?("spec") &&
         entries.include?("config")
end

spec_helper_lite_spec.rb

require_relative "../spec_helper_lite"

describe "test_helper_lite" do
  it "knows when directory is root" do
    root_dir = File.expand_path(File.dirname(__FILE__) + "/../../../") # according to the current directory structure of the app and where the current file exists
    is_root?(root_dir).should be_true
  end

  it "knows when directory is not root" do
    not_root_dir = File.dirname(__FILE__) 
    is_root?(not_root_dir).should be_false 
  end

  it "goes one level back to find the root" do
    current_directory = File.expand_path(File.dirname(__FILE__) + "/../")
    root_dir = find_rails_root(current_directory)
    is_root?(root_dir).should be_true
  end

  it "goes back as many level as needed to find the root directory" do
    current_directory = File.dirname(__FILE__)
    root_dir = find_rails_root(current_directory)
    is_root?(root_dir).should be_true
  end
end