June 12, 2006
Rails Testing Strategies: Positive and Negative Controls
Testing your rails application can be a great way to make your application robust. Proper testing aids in debugging, and can alert you when some minor change you made breaks something horribly. Rails has some nice testing features ‘baked-in’, but the best advice I have seen so far regarding which actual tests to run is … ‘it depends’.
Needless to say, that isn’t very practical advice.
Over the course of developing my own rails application, I have developed a list of ‘tests’ that I like to run on my models and controllers. This article will discuss some of the strategic design of tests, but won’t cover how to implement them (well, not much anyway).
Positive Controls
Tests should be designed with both ‘positive’ and ‘negative’ controls in mind. A positive control is a test or assertion that succeeds when you expect it to, and likewise a negative control fails when you expect it to. For example, if you create and save an object using predefined good data, an appropriate positive control could be…
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def test_create
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num_items = Item.count
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@item = Item.new
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assert @item.save, "Item not saved"
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assert_valid @item # a positive control
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assert_equal num_items + 1, Item.count #another positive control
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end
In this case, we just check to make sure that the object is valid and that the number of them in the database has increased by one. These types of tests don’t really tell you much. The only time these types of tests fail is when something is seriously wrong with your database server, your ruby install, or your rails install. The only other time this type of test will fail is if you change the definition of the ‘Item’ model so that creating one using the ‘new’ method results in an invalid object (more on that later). The problem with positive controls is that when they pass, all they tell you is that nothing in the test caused an assertion to fail. This could mean that everything is hunky-dory, or it could mean that your assertions are not working as expected.
Passing the test doesn’t mean much if the test itself can’t detect a failure. Our test would pass just fine, even if the object model should really have a ‘validates_presence_of :name’ specified. It’s also easy to see that if we write a test like this…
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def test_name
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@object = Object.new(:name=>’test name’)
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@object.save
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assert_valid @object
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assert_equal @object.name, ‘test name’
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end
This test will still pass, even if the validation is not specified. Still, it’s nice to know that things work as expected.
Negative Controls
Negative controls are really much more handy. A Negative control is a test that is designed to detect when things fail.
If we write another test for our ‘item’
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class Item < ActiveRecord::Base
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validates_presence_of :name
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end
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def test_valid
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@item = Item.new
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@item.name = "test item"
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assert_valid @item
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end
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def test_invalid
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@item = Item.new
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assert !@item.valid?, "Object is not invalid"
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end
If ‘test_invalid’ passes, then you can be confident that your ‘validates_presence_of’ is working. If it had been commented out during a bug fix and never replaced, this test would fail, while ‘test_valid’ would still happily report a success.
The same approach should be used when testing model methods. At least one test for each type of expected output, and one for each possible failure mode. Then make sure that the negative controls are in place so that they will pass when the model is fed corrupt data.
The bottom line… tests should be written to pass when you pass valid data, but they should also be written to pass when you pass missing, corrupt, or extra data. The failure modes are the most useful during development since they will point out places where bugs and security holes may creep into your application.
Filed by Kevin Olbrich at 1:36 pm under Ruby on Rails, testing
5 Comments
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