I have a Rails project which I neglected to build tests for (for shame!) and the code base has gotten pretty large. A friend of mine said that RSpec was a pain to use unless you use it from the beginning. Is this true? What would make him say that?
So, considering the available tests suites and the fact that the code base is already there, what would be my best course of action for getting this thing testable? Is it really that much different than doing it from the beginning?
Maybe start with the models? They should be testable in isolation, which ought to make them the lowest-hanging fruit.
Then pick a model and start writing tests that say what it does. As you go along, think about other ways to test the code - are there edge cases that maybe you're not sure about? Write the tests and see how the model behaves. As you develop the tests, you may see areas in the code that aren't as clean and de-duplicated (DRY) as they might be. Now you have tests, you can refactor the code, since you know that you're not affecting behaviour. Try not to start improving design until you have tests in place - that way lies madness.
Once you have the models pinned down, move up.
That's one way. Alternatives might be starting with views or controllers, but you may find it easier to start with end-to-end transaction tests andwork your way into smaller and smaller pieces as you go along.
This question came up recently on the RSpec mailing list, and the advice we generally gave was:
You may find that the design of code which wasn't driven out by code examples or unit tests makes it awkward to write tests or specs for. This is perhaps what your friend was alluding to. You will almost certainly need to learn a few key refactoring techniques to break up dependencies so that you can exercise each class in isolation from your specs. Michael Feathers' excellent book, Working Effectively With Legacy Code has some great material to help you learn this delicate skill.
I'd also encourage you to use the built-in spec:rcov rake task to generate code coverage stats. It's extremely rewarding to watch these numbers go up as you start to get your codebase under test.
The accepted answer is good advice - although not practical in some instances. I recently was faced with this problem on a few apps of mine because I NEEDED tests for existing code. There simply was no other way around it.
I started off doing all unit tests, then moved onto functionals.
Get in the habit of writing failing tests for any new code, or whenever you're going to change a part of the system. I've found this has helped me gain more knowledge of testing as I go.
Use rcov to measure your progress.
Good luck!
Writing tests for existing code may reveal bugs in your code. These tests will force you to look at the existing code so you can see what test you need to write in order to get it to pass and you may see some code that could possibly be written better, or is now useless.
Another tip is to write a test when you encounter a bug so it should never re-occur, this is called regressional testing.
Retrofitting specs is not inevitably a bad idea. You go from working code to working code with known properties which allows you to understand whether any future change breaks anything. At the moment if you need to make a change how can you know what it will affect?
What people mean when they say that it is hard to add tests/specs to exisitng code is that code which is hard to test is often highly coupled which makes it hard to write low-level isolated tests.
One idea would be to start with full-stack tests using something like the RSpec story runner. You can then work from the 'outside in' isolating what you can in low-level isolated tests and gradually untangle the harder code bit by bit.