Introduce assert_changes and assert_no_changes

[gsamokovarov]

Those are assertions that I really do miss from the standard
ActiveSupport::TestCase. Think of those as a more general version of
assert_difference and assert_no_difference (those can be implemented
by assert_changes, should this change be accepted).

Why do we need those? They are useful when you want to check a
side-effect of an operation. assert_difference do cover a really
common case, but assert_changes gives us more control. Having a
global error flag? You can test it easily with assert_changes. In
fact, you can be really specific about the initial state and the
terminal one.

error = Error.new(:bad)
assert_changes -> { Error.current }, from: nil, to: error do
  expected_bad_operation
end

assert_changes follows assert_difference and a string can be given
for evaluation as well.

error = Error.new(:bad)
assert_changes 'Error.current', from: nil, to: error do
  expected_bad_operation
end

Check out the test cases if you wanna see more examples.

🍻

[rails-bot]

r? @chancancode

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[lsylvester]

It might be useful to use === for comparing the to and from values, so that you can use classes, regexps, procs, etc when the exact value of the output is not known. i.e.

assert_changes -> { user.token }, to: /\w{24}/  do
  put :regenerate_token
end

assert_changes -> {article.published_at}, from: nil, to: Time do
  put :publish, params: {id: article.id}
end

[rafaelfranca]

👍 for @lsylvester comment.

Other than that this seems good to me. @gsamokovarov could you update the PR?

[gsamokovarov]

Sure, will look it up in a bit.

[gsamokovarov]

@rafaelfranca updated the PR with case operator support. Thanks for the idea, @lsylvester!

[dmitry]

Is there are a gem that provides similar functionality to the older rails apps?

[gsamokovarov]

I can backport it in a gem for sure.

[dmitry]

@gsamokovarov would be great to have this methods as separate gem, as I would like to use assert_changes in the old project (rails 3.2). Or if it could be backported to the rails 3.2 would be also great, but I don't think core guys will allow to do that :)

[gsamokovarov]

It's a new feature, it can't be backported to any other version. I'll get the gem out shortly.

[gsamokovarov]

@dmitry add the following in your Gemfile and you should be good.

group :test do
  gem 'minitest-assert_changes'
end

🍻

[dmitry]

@gsamokovarov thanks for gem!