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How to Use MiniTest


MiniTest is the preferred method of testing in Ruby 1.9.3 and up. Ruby is currently at version 2.1.2, so if you haven’t taken a look at MiniTest just yet, I recommend checking it out. There are some compelling features of MiniTest that make it a viable option as a testing library:

Test::Unit Compatibility
MiniTest supports assertions and test cases just like its predecessor Test::Unit. If an existing test suite is written in Test::Unit, conversion is relatively easy and painless.

Expectation Syntax
In addition to the assert syntax, MiniTest supports an expectation syntax similar to RSpec. The expectation syntax is largely a preference for each individual developer (or team), and there is no need to pull in an external library.

Test Benchmarking
MiniTest supports performance benchmarking out of the box. As the documentation states, “you can assert that your newb co-worker doesn’t replace your linear algorithm with an exponential one!”

Mocking and Stubbing
Mocking and stubbing are built-in to MiniTest! There’s no need to add in any separate libraries.

Writing Your First Test

In this section, we’re going to write a simple class and a test for it. This will give us an idea of how MiniTest works.

Let’s take a look at following simple class (blog.rb):

class Blog
  def title
    "Treehouse Blog"

Writing a test for it is simple using MiniTest. Here’s blog_test.rb:

require 'minitest/autorun'
require './blog'

class TestBlog < Minitest::Test

  def setup
    @blog =

  def test_title_is_treehouse
    assert_equal "Treehouse Blog", @blog.title

We run it the following way:

ruby blog_test.rb

And you should see:

$ ruby blog_test.rb
Run options: --seed 30102

# Running:


Finished in 0.000980s, 1020.4082 runs/s, 1020.4082 assertions/s.

1 runs, 1 assertions, 0 failures, 0 errors, 0 skips

Writing Your First Expectation

MiniTest also supports the option of writing tests in the expectation style. If you’ve used RSpec, the syntax is similar and you’ll feel right at home with describe and it blocks. Here’s what the same test would look like when writing as an expectation (blog_spec.rb):

require 'minitest/autorun'
require './blog'

describe Blog do
  before do
    @blog =

  describe "#title" do
    it "returns the title of the blog" do
      @blog.title.must_equal "Treehouse Blog"

Run it the same way:

$ ruby blog_spec.rb
Run options: --seed 19668

# Running:


Finished in 0.001998s, 500.5005 runs/s, 500.5005 assertions/s.

1 runs, 1 assertions, 0 failures, 0 errors, 0 skips

Supported Assertions and Expectations

There are a ton of different assertions that are supported in Minitest. Some of the assertions that you’ll be using most often are:

Assertion Expectation
assert_equal must_equal
assert_instance_of must_be_instance_of
assert_nil must_be_nil
assert_raises must_raise

See the documentation for a complete list of assertions and expectations.

MiniTest and Rails

Using MiniTest with Rails is completely supported. It takes a slight bit of setup to get working, however, it is just as easy as using something like RSpec. There are a couple of different options when using MiniTest with a Rails application. The first one we’ll take a look at is minitest-rails. Setup is straightforward in this library. To install, open up your Gemfile and place the following:

  gem "minitest-rails"

Then install it:

  bundle install
  rails generate minitest:install

It’s also possible specify using the expectation format by default in config/application.rb:

config.generators do |g|
  g.test_framework :minitest, spec: true

Your application will now have minitest files generated by default when you run the rails generate command. Easy!

Another library worth looking at is minitest-spec-rails. This library accomplishes the same thing as the minitest-rails gem. However, minitest-spec-rails has slightly different priorities which you can read about in the Readme. Either library works, and I encourage you to read both project’s documentation and decide which may be a better fit for your project.

Try It!

There are a wealth of testing options in the Ruby community. For a while, it seemed like a new testing library popped up every day. If you’re going to be doing Ruby development, it is important to be familiar with the standard testing library that ships with the language. If you haven’t given MiniTest a try, check it out! You just may like what you see.

Update: Ryan Davis has pointed out that in MiniTest 4, tests should inherit from MiniTest::Unit::TestCase and in MiniTest 5, tests should inherit from Minitest::Test. The code in the article has been updated to reflect this. Thanks, Ryan!

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