I was TDDing a new website that I’ve been working on last night and got caught in the interesting predicament where the tests failed, but the production code worked. For the life of me, I could not figure out why my test was failing. It turns out, I missed a tiny little piece of documentation on how shoulda works.

By the way, before we get into this, if you are writing Ruby code and writing tests (you are, arent you?), do yourself a favor and check out the shoulda library. Excellent work from the great folks at thoughtbot.

context "with valid attributes" do
  setup do
    @user = Factory.create(:user)
    @updated_attributes = Factory.attributes_for(:user)

    put :update, :id => @user.id, :user => @updated_attributes
  end

  should_not_change "User.count"
  should_respond_with :success
  should_redirect_to 'root_url'
end

For quite some time, every test was passing with the exception of should_not_change "User.count". After consulting the documentation and source code for shoulda, I realized what should_not_change was actually doing.

The should_not_change macro was evaluating the User.count statement PRIOR to the setup method executing and stored the result in a variable. Then when the test executes, it evaluated the User.count statement again. Since the Factory.create call in the setup method created a new instance in the database, of course, User.count would change.

To get around this particular example, I ended up having to create a nested context to make the test pass. I dont necessarily like this, but it does get the test to pass and gives me an opportunity to change it if someone has a better solution.

context "updating User information" do
  setup do
    @user = Factory.create(:user)
    @updated_attributes = Factory.attributes_for(:user)
  end

  context "with valid attributes" do
    setup { put :update, :id => @user.id, :user => @updated_attributes }

    should_not_change "User.count"
    should_respond_with :success
    should_redirect_to 'root_url'
  end
end

Indeed, sometimes the problem lies in your tests.