Minitest Testing for Rails

Write comprehensive, maintainable Minitest tests following Rails conventions and best practices. This skill provides patterns for model tests, controller tests, system tests with Capybara, and fixture usage.

Core Testing Principles

1. Test Structure

Organize tests with clear phases:

test "creates article with valid attributes" do

Arrange - set up test data

user = users(:alice)

Act - perform the action

article = Article.create( title: "Test Article", body: "Content here", user: user )

Assert - verify the outcome

assert article.persisted? assert_equal "Test Article", article.title end

2. Test Independence

Each test should be independent and able to run in any order. Use setup for common initialization:

class ArticleTest < ActiveSupport::TestCase setup do @user = users(:alice) @article = articles(:published) end

test "publishes article" do @article.publish! assert @article.published? end end

3. Single Responsibility

Each test verifies one behavior, though it may use multiple assertions to fully verify that behavior.

Test Types

Model Tests (test/models/ )

Test business logic, validations, associations, and custom methods.

File location: test/models/article_test.rb

class ArticleTest < ActiveSupport::TestCase test "validates presence of title" do article = Article.new(body: "Content") assert_not article.valid? assert_includes article.errors[:title], "can't be blank" end

test "belongs to user" do article = articles(:published) assert_instance_of User, article.user end

test "#published? returns true when status is published" do article = articles(:published) assert article.published? end end

Controller Tests (test/controllers/ )

Test HTTP responses, redirects, authentication, and parameter handling.

File location: test/controllers/articles_controller_test.rb

class ArticlesControllerTest < ActionDispatch::IntegrationTest setup do @user = users(:alice) sign_in_as(@user) # helper method end

test "GET index returns success" do get articles_path assert_response :success end

test "POST create with valid params creates article" do assert_difference("Article.count", 1) do post articles_path, params: { article: { title: "New Article", body: "Content" } } end

assert_redirected_to article_path(Article.last)

end

test "POST create with invalid params renders new" do assert_no_difference("Article.count") do post articles_path, params: { article: { title: "", body: "" } } end

assert_response :unprocessable_entity

end end

System Tests (test/system/ )

Test end-to-end user workflows with browser simulation.

File location: test/system/article_creation_test.rb

class ArticleCreationTest < ApplicationSystemTestCase test "user creates new article" do visit root_path click_on "New Article"

fill_in "Title", with: "My Test Article"
fill_in "Body", with: "This is the content"
select "Published", from: "Status"

click_on "Create Article"

assert_text "Article was successfully created"
assert_text "My Test Article"

end

test "article updates with Turbo Frame" do article = articles(:draft) visit article_path(article)

click_on "Edit"
fill_in "Title", with: "Updated Title"
click_on "Update Article"

# Turbo Frame update - no full page reload
assert_text "Updated Title"
assert_no_text article.title

end end

Detailed Patterns & Examples

For comprehensive Minitest patterns and examples, see examples.md.

Pattern Categories

examples.md contains:

• Fixtures - YAML fixtures, associations, and fixture access

• Model Testing - Validations, associations, scopes, callbacks, enums

• Controller Testing - CRUD actions, authentication, authorization

• System Testing - Capybara selectors, form interactions, JavaScript behavior

• Turbo/Hotwire Testing - Turbo Frames, Turbo Streams, Stimulus controllers

• Background Jobs - Testing ActiveJob and SolidQueue jobs

• Mailers - Testing ActionMailer delivery and content

• Test Helpers - Custom assertions and helper methods

• Mocking/Stubbing - When and how to use mocks with Minitest

Refer to examples.md for complete code examples.

Common Assertions

Equality

assert_equal expected, actual assert_not_equal unexpected, actual

Truth/Falsehood

assert value assert_not value assert_nil value assert_not_nil value

Predicates

assert article.valid? assert_not article.persisted? assert articles.empty?

Collections

assert_includes array, item assert_empty collection assert_not_empty collection

Differences (counting changes)

assert_difference "Article.count", 1 do Article.create!(title: "New", body: "Content") end

assert_no_difference "Article.count" do Article.create(title: "", body: "") # invalid end

HTTP Responses

assert_response :success assert_response :redirect assert_response :not_found assert_redirected_to articles_path

Errors

assert_raises ActiveRecord::RecordInvalid do Article.create!(title: nil) end

Fixtures

Fixture Files (test/fixtures/*.yml )

test/fixtures/users.yml

alice: email: alice@example.com name: Alice Smith role: admin

bob: email: bob@example.com name: Bob Jones role: member

test/fixtures/articles.yml

published: user: alice title: Published Article body: This article is live status: published published_at: <%= 1.day.ago %>

draft: user: bob title: Draft Article body: Work in progress status: draft

Using Fixtures in Tests

test "finds published articles" do published = articles(:published) draft = articles(:draft)

results = Article.published

assert_includes results, published assert_not_includes results, draft end

Test Helpers

Authentication Helper

test/test_helper.rb

class ActionDispatch::IntegrationTest def sign_in_as(user) post login_path, params: { email: user.email, password: "password" } end end

Custom Assertions

test/test_helper.rb

module ActiveSupport class TestCase def assert_valid(record) assert record.valid?, "Expected #{record.class} to be valid, errors: #{record.errors.full_messages}" end end end

Running Tests

Run all tests

bin/rails test

Run specific test file

bin/rails test test/models/article_test.rb

Run specific test by line number

bin/rails test test/models/article_test.rb:12

Run tests by pattern

bin/rails test test/models/*_test.rb

Run system tests only

bin/rails test:system

Run tests in parallel

bin/rails test:parallel

Run with verbose output

bin/rails test -v

Run and show coverage

COVERAGE=true bin/rails test

Official Documentation:

• Rails Testing Guide - Comprehensive Rails testing guide

• Minitest Documentation - Minitest framework docs

• Capybara Cheat Sheet - System test selectors

Tools:

• SimpleCov - Code coverage for Ruby

• shoulda-matchers - Additional matchers for Rails