I've been banging my head against the wall that is ActiveResource for a while now. One big problem is actually getting the testing correct.
Take 1: HTTPMock
The supposedly sanctioned expectation is that you test it against the provided HTTPmock... unfortunately, this is great for testing that ActiveResource makes the correct remote calls eg, when you update a user that ARes POSTs to /users/123.xml... and that it reacts to a (pre-stubbed) 404 by raising a ResourceNotFound; but it doesn't allow you to actually test your model - eg to assert that when you update your user's login field and reload your model, that your user can now login with the new details...
AFAICS you can't test anything that is actually meaningful or useful to your business logic - which kinda defeats the point IMO.
Even with a lot of hacking, I couldn't get the kind of dynamic mock backend that I needed for any meaningful test-coverage.
Take 2: Mocking with Stump
Stump let me do much more dynamic testing. I could stub out the "create" and "find" functions and make it respond with a mock remote object that I could then further stub out the "save" and "update" functions... and after a while it seemed like I was stubbing out functions to return stubbed out stubs and it all got a little bit circular... and in many cases: really complex.
I also found that I got the code to work in the browser I'd often still have to spend ages getting the tests to work with the right set of mocks and stubs to patch all the ways that rails could leak out and try to call the "real" API. :P
I also noticed a few times where I'd somehow accidentally plastered over a bug with a stub by assuming the return-value. I only noticed this because the tests would run fine - yet trying the same operation in the browser revealed the bug.
Now how can I test if I can't rely on the test to actually test? My safety net suddenly has big gaping holes in it that I'll only find if I fall through them while testing by hand! This is such a step backwards that it's horrifying!
Then I hit a wall.
We have a user and during creation (and update) we need to check that the login is unique - which requires a find on the remote API. So in the unit tests we mock out the User object's "find" method to return an empty set... (ie no users were found that match this login field). But then (immediately after creation), we need to be able to find the user - we we re-stub out User.find to return the given user - otherwise the test fails.
... but when you're doing a functional test, all of the above operations are inside the atomic post :create call. You can't stub out one half, then stub out the other half of the operation partway through... because you can't get to the user object partway through the process... so creates were suddenly failing badly with seemingly no solution.
The code didn't do what I needed, and on top of this was tangled and messy and really too ugly. When it comes to rails, ugly code screams out for another solution...
Take 3: A real API
So, I turned to (what I see as) the lesser of two evils. I have created a test-API project that will simulate the real remote API with the models that are used by the (local) project. the test environment calls this API instead of the "real" one by using a defined constant for the site.
In setup I send the ActiveResource model a delete_all which will clear the remote db. This simulates what Rails testing does anyway (ie clearing out the db before each test).
So now the project tests against a real, live-but-fake API that I can run in another window on my local machine. The tests run - they look pretty much the same as my ActiveRecord tests - which means I can check everything I need to check. It also means I can be assured that the code hasn't plastered over a bug with mocks and stubs.
- Real end-end tests
- Dynamic remote object instead of static mocks (means you can test that values are changed and that reloading returns what you expect)
- Fewer assumptions means the test-code is less likely to be buggy - ie you're more likely to be actually testing what you think you're testing.
- Easier to test as you don't have to mock out things that will return things that you've mocked...
- Above leads to a more natural Railsy test syntax.
- Keeping two projects in synch
- extra development time of the fake API
- Will need to make sure the API a) exists and b) is running before you can run tests
- Have to verify that the mock API does what the real API does.
- Tests run slower as there's the (local) network turnaround time to consider.
In my mind, the pros outweigh the cons. YMMV
 We're currently not using fixtures anyway, but it we were - I would then send the remote API a "please load up your fixtures" command.