Tag Archive for 'Ruby (on Rails)'

Update: This code and documentation is now available on github: http://github.com/ndp/assert_changes/tree/master

The Problem

On our work on gobalto.com, we spend time to have good fixture data for our tests– data that can represent all the important application states that our tests require. As a result, our tests are very dependent on the data. It’s important that someone doesn’t inadvertantly change it in subtle ways. This has led us to write not only asserts at the end of tests, but pre-conditions as well. For example,

    ...
    inotech = companies(:inotech)
    assert inotech.services.public.include?(categories(:a))
    assert inotech.services.public.include?(categories(:b))
    assert inotech.services.public.include?(categories(:c))
 
    post :edit_services_dialog, :id=>inotech.id,
                                          :service_category_id=>categories(:a).id
    inotech.reload
 
    assert inotech.services.public.include?(categories(:a))
    assert !inotech.services.public.include?(categories(:b))
    assert !inotech.services.public.include?(categories(:c))

Although the pre-conditions were introduced to guard against accidentally changing fixture data (or just to figure out what’s going on), we don’t necessarily delete them. They provide stronger tests. And in some ways, leaving in pre-conditions make the code more readable by providing documentation of your assumptions to the readers. The code above would be hard to follow without the clarifying pre-conditions. What do we expect to change and what stays the same?

Unfortunately all these asserts make the tests twice as long. And there’s a subtle readability problem: there’s no relationship between the corresponding pre- and post-conditions. In the example above, you have to scan carefully to see that the three assertions are repeated, but negated (b and c only). The reader must mentally put pieces together. And it’s not DRY. Using local variables doesn’t help much.

The Solution

I was inspired by a nice little test helper called assert_difference. It takes a string to evaluate and a block to execute. It’s useful for checking on state changes– especially database changes– during a test:

assert_difference('Company.count', -1) do
    Company.delete_one
end

(Without this method, we rely on a count of all database records, or merely look for specific ones. The former approach leads to brittle tests, and the latter to incomplete assertions.)

A limitation of assert_difference is that it only deals with integers. What if it were generalized? Here goes:

    i = true
    assert_changes 'i' => false do   # read as: i changes to false
      i = false
    end

Continue reading ‘assert_changes and assert_no_changes in Ruby’