Minitest Coder

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: Spend coverage where regressions would hurt, not where framework behavior is already proven
• Fast Tests: Minimize database operations
• Fixtures: Use fixtures for test data
• No Test-Induced Design Damage: Never add production indirection only to make a test easier

Coverage Budget

• Heavy: model tests for domain invariants and controller/integration tests for full request flows, authorization, formats, and state changes.
• Focused: system tests for critical happy paths and cross-browser behavior that lower layers cannot prove.
• Light: jobs, mailers, broadcasts, and channels unless they contain real branching or security-sensitive behavior.
• Avoid: view tests, trivial delegation tests, and tests that only assert Rails framework behavior.

Do not duplicate the same behavior assertion at every layer. Pick the lowest layer that proves the behavior and add an integration test when wiring or authorization matters.

Minitest Styles

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
• test/services/ - Service object tests, only when the app has real service objects
• test/integration/ - Full-stack tests
• test/mailers/ - Mailer tests
• test/jobs/ - Background job tests
• test/fixtures/ - Test data
• test/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 references/spec-patterns.md for detailed examples.

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 references/spec-patterns.md for detailed examples.

Stub external boundaries such as network calls, time, payment providers, object storage, and randomness. Avoid mocking internal application methods just to prove a private call happened.

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

Detailed Reference

Load detailed-reference.md for extended examples, templates, and advanced patterns.