I was reading Google Groups' Rails Group the other day when I ran into someone talking about why should I test the validates functions. Here is a quote from his email:

For example, what is the point of writing a unit test that simply tests a validate statement in the model? Yes, it's interesting (the first time, at least) that the test works and that, golly-gee, Rails works as advertised, as well, but is there any real use in doing this?
I can imagine a scenario where a I update Rails and suddenly a unit test that tests a Rails validator fails, but I expect the Rails development team would find this before me. Or is that too naive?

I just thought I would post my response for anyone who has the same question. Also, sorry if the format is bad or to quick, but I was trying to hurry when I wrote him the response.

Don't look it as testing Rails. You are testing your model on testing validates. Let's say you write this class:

class Person < ActiveRecord::Base
  validates_length_of :phone_number, :within => 7..10
  ...
end

So you write a test:

def test_phone_number_length_incorrect
  person = Person.new(:phone_number => '123456')
  person.valid?
  assert person.errors.invalid?(:phone_number), 'Invalid Phone Number Coming Back as Valid'
end

I think we would all agree that this test makes sure that a valid phone number passes.

Now not thinking you do a find and replace and replace all 7s with 8s or maybe number with numbers is more likely. So you have:

class Person < ActiveRecord::Base
  validates_length_of :phone_numbers, :within => 7..10
  ...
end

Now you can place :phone_number => '12' into your database because you are now validating :phone_numbers.

If you have a test you run tests after you completed the find and replace, and this error would be caught right away. That is why you 'test Rails.'