stubbing File.expand_path in rspec/rails 4

Question

i have multiple calls to File.expand_path(Rails.root) in my code

for testing i created following configuration in spec/support

RSpec.configure do |config|
  config.before(:each) do
    File.stub(:expand_path).and_return("spec/fs")
  end
end

so that instead of "/home/user/" each request to File.expand_path returns "spec/fs/"

it worked well while i was on rails 3

however after moving to rails 4, tests started throwing following error:

 Failure/Error: let(:category) { build(:category) }
 LoadError:
   cannot load such file -- spec/fs

why does it appear/how can i fix that?

ps.

tests/models are very basic, but strangely this test case fails:

#in class
def first method(category)
  "#{File.expand_path(Rails.root)}/public/collections/#{self.name}/_new/#{category.name.downcase}"
end

#in rspec
describe "#first_method" do
  it { expect(collection.first_method(category)).to be_instance_of(String) }
end

but this one doesn't!

#in class
def second_method
  "#{File.expand_path(Rails.root)}/public/collections/#{self.name}/_new/"
end

#in rspec
describe "#second_method" do
  it { expect(collection.second_method).to be_instance_of(String) }
end

category factory being as simple as:

FactoryGirl.define do
  factory :category do
    name "TestCategory"
  end
end

built with simple

let(:category) { build(:category) }

however it seems like stub failure occurs just while building the :category factory

Answer 1

fixed with defining File.expand_path(Rails.root) as application-wide environment-dependant constant, and modifying it for test environment.

Answer 2

File.expand_path is a very commonly used method. I would not be surprised if the Rails, RSpec or FactoryGirl software relied on it as part of their code which gets executed after the double is put in place (e.g. to load the Category model code). You could easily verify that by printing a stack trace or going to the debugger when it gets invoked and seeing where you are.