Behavior Driven Development using existing software components

Question

I'm currently reading through the beta version of The Rspec Book: http://www.pragprog.com/titles/achbd/the-rspec-book

It describes The Behaviour Driven Development Cycle (red, green, refactor) as taking small steps during the development process. This means adding features one at a time.

My question is:

If I were to describe a single feature of my software (eg: a successful user login scenario in a cucumber test) AND if I were using a modular component (such as Devise) which has many features (scenarios). How could I possibly follow Behaviour Driven techniques? Once my first step passes, I must reverse engineer my other tests to reflect the functionality of the software component I am using thus violating the principal of BDD!

Edit (for clarity):

My first scenario is passing after implementing Devise. But now I must factor all my subsequent end to end tests (that I have not yet written) around the behaviour of Devise rather than to my stake holder's requirements. So the BDD cycle can no longer be applied. I must reverse engineer Devise in my tests to make them pass or not write the tests.

Answer 1

So I can understand better: You have an existing system and it includes tests but does not include login functionality. You decide to add login:

Given a visitor is not logged in
When a visitor goes to the admin page
Then the visitor should see the login page

Ok, so then you decide how you want to implement the login because this scenario is failing. Right? So you choose Devise and the scenario passes but all the tests or specs that relied on a security-free system now fail. Am I still on track? If that's the case, you've encountered a case where you are adding a feature that is so pervasive in your application that you need to touch a number of tests/specs to get them all running. You are still testing first because either the code is broken or the test doesn't recognize the new feature. In any case, you know where the work needs to be done.

Helpful?