Ruby: Modifying Class Attributes in Cucumber and RSpec

Imagine you're building Amazon.com, and you have a rating system. You might not want to show an average rating to the user until there's been some minimum number of ratings. Of course, you'll want to test that that feature works. However, in other tests, you might want to disable that feature by setting the minimum number of ratings required to 0.

More generally, I sometimes run into the situation where I have a constant in my model that I might want to change, but only for a specific test. Instead of using a constant, you can use a class attribute. However, since class attributes aren't part of a database transaction, they don't get reset after every test. That's a problem since one test that changes the class attribute might mysteriously break another test that relies on that class attribute. I had a problem where one feature file would pass when run alone, but it wouldn't pass when all the features files were run. Here's how to make it all work.

In your Rating model, create your class attribute as follows:

# Rating.min_num_ratings is the minimum number of ratings before
# we show the ratings average to the user.  We don't want a single bad
# rating to forever scar a book's reputation.
#
# I'm setting this up as a class accessor so that you can set it to 0 in your
# tests.  Beware that Cucumber and RSpec don't reset this after every test
# since this isn't part of a database transaction.  Hence, if you change
# Rating.min_num_ratings, you must change it back after your test.  In
# RSpec, you can do this with a begin/ensure block.  In Cucumber, you can use
# the @afterwards_reset_min_num_ratings annotation.
DEFAULT_MIN_NUM_RATINGS = 3
cattr_accessor :min_num_ratings
self.min_num_ratings = DEFAULT_MIN_NUM_RATINGS

Your RSpec tests should look like:

it "should do something useful when the minimum number of ratings has been hit" do
  Rating.min_num_ratings = 2
  begin
    # Test that it does something useful.
  ensure
    Rating.min_num_ratings = Rating::DEFAULT_MIN_NUM_RATINGS
  end
end

Just to be sure I didn't forget to reset min_num_ratings anywhere, I have the following at the bottom of my .spec file:

it "should still have the default min_num_ratings" do
  Rating.min_num_ratings.should == Rating::DEFAULT_MIN_NUM_RATINGS
end

Cucumber is a bit harder since you can't use a begin/ensure block. I started by adding a step:

# If you use this step, you *must* use the @afterwards_reset_min_num_ratings
# annotation on your scenario in order to reset Rating.min_num_ratings after
# your test.
Given /^the min_num_ratings has been set to (\d+)$/ do |min|
  Rating.min_num_ratings = min.to_i
end

Then, I made use of the step and the annotation in my tests:

@afterwards_reset_min_num_ratings
Scenario: users can vote, and their vote affects the average
  Given there is only one book
  And the min_num_ratings has been set to 2
  And a user has already given the book 2.0 stars
  And I am logged in as a user
  When I follow "Moby Dick"
  And I choose by xpath "//input[@name='rating' and @value='1.0']"
  And I press "Rate"
  Then I should see "Moby Dick"
  And I should see "Thanks for rating!"
  And the CSS class for the flash message should be "success"
  And I should see an average rating of 1.5 stars

Last of all, I created a new file, features/support/hooks.rb, with the following:

# This will only run after scenarios tagged with
# @afterwards_reset_min_num_ratings.
After("@afterwards_reset_min_num_ratings") do
  Rating.min_num_ratings = Rating::DEFAULT_MIN_NUM_RATINGS
end

I know this is a bit esoteric, but this same approach can be used anytime you have just a few tests that require test-specific configuration.