Writing a Basic Minitest Test

Direct Answer

A Rails Minitest test inherits from the appropriate Rails test class, defines a clearly named test block, prepares the input, calls the public behaviour, and checks the result with a specific assertion. Model tests commonly inherit from ActiveSupport::TestCase; integration tests use ActionDispatch::IntegrationTest. Assertions normally put the expected value first, as in assert_equal 118, invoice.total, and each test must be independent.

Deep Dive & Architecture

Rails selects test helpers through inheritance. ActiveSupport::TestCase is suitable for models and plain application objects that need Rails support. ActionDispatch::IntegrationTest adds HTTP request methods and response access. System tests add a browser driver. Choosing the smallest boundary keeps setup and failure output focused.

Assertions are executable claims. assert_equal compares values, assert_includes checks membership, and assert_changes can verify a Rails state transition around a block. A generic assert result may pass for the wrong truthy value, so use an assertion that names the real promise whenever one exists.

Rails maintains a separate test database and normally isolates database changes between tests. That does not automatically isolate global variables, files, network services, clocks, or jobs. Those boundaries still need controlled data, fakes, or cleanup suited to the behaviour.

Test code is production support code: duplication can be extracted, but clarity comes first. A small local setup that exposes the scenario is often better than an abstract helper with several flags. When a failure occurs, the test name, input, assertion, and message should reveal what contract was broken.

Code Example

require 'minitest/autorun'

class Price
  def self.total(subtotal, tax_rate:)
    subtotal + (subtotal * tax_rate)
  end
end

class PriceTest < Minitest::Test
  def test_total_applies_the_tax_rate
    assert_equal 118, Price.total(100, tax_rate: 0.18)
  end
end

Common Mistakes & Pitfalls

Using RSpec-style expect in Minitest (use assert) — or not knowing Minitest is Ruby's default.