Minitest & TDD Best Practices

Minitest is Rails' default testing framework. This skill covers TDD workflow, Minitest patterns, and testing best practices for Rails applications.

TDD Workflow: Red, Green, Refactor

Red: Write failing test Green: Write minimum code to pass Refactor: Improve code with safety net

Test Types

Model Tests (Unit):

class OrderTest < ActiveSupport::TestCase
  test "calculates total correctly" do
    order = orders(:standard)
    assert_equal 100.0, order.total
  end
end

Controller Tests (Functional):

class PostsControllerTest < ActionDispatch::IntegrationTest
  test "GET index returns success" do
    get posts_path
    assert_response :success
  end
end

Integration Tests (System):

class UserFlowsTest < ActionDispatch::IntegrationTest
  test "user can create post" do
    post users_path, params: { user: { email: "test@example.com" } }
    post posts_path, params: { post: { title: "Test" } }
    assert_equal "Test", Post.last.title
  end
end

Minitest Assertions

assert_equal expected, actual
assert_not_equal not_expected, actual
assert condition
assert_not condition
assert_nil value
assert_includes collection, item
assert_raises ExceptionClass do
  # code
end
assert_difference "Model.count", 1 do
  # code
end

Fixtures

# test/fixtures/posts.yml
one:
  title: "First Post"
  author: steve
  published: true

test "can access fixture" do
  post = posts(:one)
  assert_equal "First Post", post.title
end

What NOT to Test

❌ Framework behavior (ActiveRecord saves, validations work) ❌ Third-party libraries ❌ Simple getters/setters

✅ Business logic ✅ Custom validations ✅ Complex workflows ✅ Edge cases

Best Practices

• One assertion per test (when possible)
• Clear test names
• Fast tests (< 1 second each)
• Independent tests (no shared state)
• DRY tests (extract common setup)

Test your code, not the framework.

Additional Resources

Example Files

Working examples in examples/:

• examples/model_test_example.rb - Comprehensive model test demonstrating validations, associations, business logic, edge cases, state transitions, scopes, and callbacks

External Resources

• Minitest Documentation
• Rails Testing Guide