Minitest Coder

Core Philosophy

• AAA Pattern: Arrange-Act-Assert structure for clarity
• Behavior over Implementation: Test what code does, not how
• Isolation: Tests should be independent
• Descriptive Names: Clear test descriptions
• Coverage: Test happy paths AND edge cases
• Fast Tests: Minimize database operations
• Fixtures: Use fixtures for test data

Minitest Styles

test "description"

describe

it

let

subject

Traditional Style

class UserTest < ActiveSupport::TestCase
  test "validates presence of name" do
    user = User.new
    assert_not user.valid?
    assert_includes user.errors[:name], "can't be blank"
  end
end

Spec Style

class UserTest < ActiveSupport::TestCase
  describe "#full_name" do
    subject { user.full_name }
    let(:user) { User.new(first_name: "Buffy", last_name:) }

    describe "with last name" do
      let(:last_name) { "Summers" }

      it "returns full name" do
        assert_equal "Buffy Summers", subject
      end
    end
  end
end

Test Organization

File Structure

test/
├── models/           # Model unit tests
├── services/         # Service object tests
├── integration/      # Full-stack tests
├── mailers/          # Mailer tests
├── jobs/             # Background job tests
├── fixtures/         # Test data
└── test_helper.rb    # Configuration

Naming Conventions

• Mirror app structure:

  app/models/user.rb

  →

  test/models/user_test.rb

• Use fully qualified namespace:

  class Users::ProfileServiceTest

• Don't add

  require 'test_helper'

  (auto-imported)

Spec Style Patterns

See resources/spec-patterns.md for detailed examples.

subject { ... }

let(:name) { ... }

describe "context"

before { ... }

describe "#process" do
  subject { processor.process }
  let(:processor) { OrderProcessor.new(order) }
  let(:order) { orders(:paid_order) }

  it "succeeds" do
    assert subject.success?
  end
end

Fixtures

# test/fixtures/users.yml
alice:
  name: Alice Smith
  email: alice@example.com
  created_at: <%= 2.days.ago %>

class UserTest < ActiveSupport::TestCase
  fixtures :users

  test "validates uniqueness" do
    duplicate = User.new(email: users(:alice).email)
    assert_not duplicate.valid?
  end
end

Mocking and Stubbing

See resources/spec-patterns.md for detailed examples.

Object.stub :method, value

Minitest::Mock.new

test "processes payment" do
  PaymentGateway.stub :charge, true do
    processor = OrderProcessor.new(order)
    assert processor.process
  end
end

Assertions Quick Reference

Basic Assertions

# Boolean
assert user.valid?
assert_not user.admin?

# Equality
assert_equal "Alice", user.name
assert_nil user.deleted_at

# Collections
assert_includes users, admin_user
assert_empty order.items

# Exceptions
assert_raises ActiveRecord::RecordInvalid do
  user.save!
end

Rails Assertions

# Changes
assert_changes -> { user.reload.status }, to: "active" do
  user.activate!
end

assert_difference "User.count", 1 do
  User.create(name: "Charlie")
end

# Responses
assert_response :success
assert_redirected_to user_path(user)

AAA Pattern

test "processes refund" do
  # Arrange
  order = orders(:completed_order)
  original_balance = order.user.account_balance

  # Act
  result = order.process_refund

  # Assert
  assert result.success?
  assert_equal "refunded", order.reload.status
end

Spec style with subject:

describe "#process_refund" do
  subject { order.process_refund }
  let(:order) { orders(:completed_order) }

  it "updates status" do
    subject
    assert_equal "refunded", order.reload.status
  end

  it "credits user" do
    assert_changes -> { order.user.reload.account_balance }, by: order.total do
      subject
    end
  end
end

Test Coverage Standards

Coverage Example

class UserTest < ActiveSupport::TestCase
  # Validations
  test "validates presence of name" do
    user = User.new(email: "test@example.com")
    assert_not user.valid?
    assert_includes user.errors[:name], "can't be blank"
  end

  # Methods
  describe "#full_name" do
    subject { user.full_name }
    let(:user) { User.new(first_name: "Alice", last_name: "Smith") }

    it "returns full name" do
      assert_equal "Alice Smith", subject
    end

    describe "without last name" do
      let(:user) { User.new(first_name: "Alice") }

      it "returns first name only" do
        assert_equal "Alice", subject
      end
    end
  end
end

Advanced Patterns

See resources/advanced-patterns.md for production-tested patterns from 37signals.

Anti-Patterns

See resources/anti-patterns.md for detailed examples.

require 'test_helper'

@ivars

let

subject

assert x.include?(y)

assert_includes

Best Practices Checklist

Organization:

• Files mirror app structure
• NOT adding

  require 'test_helper'

• Using fully qualified namespace

Style Choice:

• Traditional for simple tests
• Spec for complex contexts
• Max 3 nesting levels

Test Data:

• Using fixtures (not factories)
• Using

  let

  for shared data
• Using

  subject

  for method under test

Assertions:

• Correct assertion methods
• Rails helpers (

  assert_changes

  ,

  assert_difference

  )
• Testing behavior, not implementation

Coverage:

• Happy path tested
• Sad path tested
• Edge cases covered

When to Choose Style

Traditional: Simple validations, straightforward tests, no shared setup

Spec: Multiple contexts, lazy evaluation needed, nested scenarios, reusable subject

Can mix both in the same file if it improves clarity.