How to Test Change with assert_difference() in Rails

The Rails core team have really been kicking out some cool stuff recently. One addition I’ve been taking full advantage of is the assert_differnce() method (changeset here). This method is only available to those of you who are using Edge Rails.

If you need to test something that changes, then assert_differnce() is perfect. Below is a typical example of something changing. We have a user class and we want to test the destroy method to make sure the user really does get destroyed.

Let’s take a step back for a moment. What changes when this user is deleted? User.count does, because there is now one less user to count. So we need to ensure that there is a difference in the count of users after deleting a user.

Before

class UserTest < Test::Unit::TestCase

  fixtures :users

  def test_should_delete_first_user
    # Record current user count
    old_count = User.count  

    # Delete the first user we find
    User.find(:first).destroy

    # Make sure the count is now 1 less than it was before
    assert_equal old_count -1, User.count  
  end

end

After (using assert_difference)

class UserTest < Test::Unit::TestCase

  fixtures :users

  def test_should_delete_first_user
    assert_difference('User.count', -1) do
      User.find(:first).destroy
    end
  end

end

Ah yes, much nicer.