Rails Minitest Expert

Write performant, maintainable, and non-brittle tests for Rails applications using Minitest and fixtures.

Philosophy

Core Principles:

1. Test behavior, not implementation - Focus on WHAT code does, not HOW
2. Fast feedback loops - Prefer unit tests over integration tests, fixtures over factories
3. Tests as documentation - Test names should describe expected behavior
4. Minimal test data - Create only what's necessary; 2 records == many records
5. Non-brittle assertions - Test outcomes, not exact values that may change

Testing Pyramid:

    /\     System Tests (Few - critical paths only)
   /  \    
  /____\   Request/Integration Tests (Some)
 /      \  
/________\ Unit Tests (Many - models, policies, services)

When To Use This Skill

• Writing new Minitest tests for Rails models, policies, controllers, or requests
• Converting RSpec tests to Minitest
• Debugging slow or flaky tests
• Improving test suite performance
• Following Rails testing conventions
• Writing fixture-based test data
• Implementing TDD workflows

Instructions

Step 1: Identify Test Type

Before writing, determine the appropriate test type:

Step 2: Check Existing Patterns

ALWAYS search for existing tests first:

# Find similar test files
rg "class.*Test < " test/

# Find existing fixtures
ls test/fixtures/

# Check for test helpers
cat test/test_helper.rb
cat test/support/*.rb

Match existing project conventions - consistency is more important than "best" patterns.

Step 3: Use Fixtures (Not Factories)

Fixtures are 10-100x faster than Factory Bot.

# AVOID - Factory Bot (slow, implicit)
let(:user) { create(:user) }
let(:project) { create(:project, workspace: workspace) }

# PREFER - Fixtures (fast, explicit)
setup do
  @workspace = workspaces(:main_workspace)
  @user = users(:admin_user)
  @project = projects(:active_project)
end

Fixture Best Practices:

• Create purpose-specific fixtures with descriptive names
• Use <%= %> for dynamic values and UUIDs
• Reference associations by fixture name, not ID
• Keep fixtures minimal - only include required attributes

# test/fixtures/users.yml
admin_user:
  id: <%= Digest::UUID.uuid_v5(Digest::UUID::OID_NAMESPACE, "admin_user") %>
  email: "admin@example.com"
  name: "Admin User"
  created_at: <%= 1.week.ago %>

member_user:
  id: <%= Digest::UUID.uuid_v5(Digest::UUID::OID_NAMESPACE, "member_user") %>
  email: "member@example.com"
  name: "Member User"
  workspace: main_workspace  # Reference by fixture name

Step 4: Write Test Structure

Standard Test Structure:

require "test_helper"

class ModelTest < ActiveSupport::TestCase
  setup do
    # Load fixtures - MINIMAL setup only
    @record = models(:fixture_name)
  end

  # Group related tests with comments or test naming
  
  # Validation tests
  test "requires name" do
    @record.name = nil
    refute @record.valid?
    assert_includes @record.errors[:name], "can't be blank"
  end

  # Method tests
  test "#full_name returns formatted name" do
    @record.first_name = "John"
    @record.last_name = "Doe"
    assert_equal "John Doe", @record.full_name
  end
end

Step 5: Follow Performance Guidelines

Avoid Database When Possible:

# SLOW - Creates database records
test "validates email format" do
  user = User.create!(email: "invalid", name: "Test")
  refute user.valid?
end

# FAST - Uses in-memory object
test "validates email format" do
  user = User.new(email: "invalid", name: "Test")
  refute user.valid?
end

Minimize Records Created:

# SLOW - Creates 25 records
test "paginates results" do
  create_list(:post, 25)
  # ...
end

# FAST - Configure pagination threshold for tests
# config/environments/test.rb: Pagy::DEFAULT[:limit] = 2
test "paginates results" do
  # Only need 3 records to test pagination with limit of 2
  assert_operator posts.count, :>=, 3
  # ...
end

Avoid Browser Tests When Possible:

# SLOW - Full browser simulation
class PostsSystemTest < ApplicationSystemTestCase
  test "creates a post" do
    visit new_post_path
    fill_in "Title", with: "Test"
    click_on "Create"
    assert_text "Post created"
  end
end

# FAST - Request test (no browser)
class PostsRequestTest < ActionDispatch::IntegrationTest
  test "creates a post" do
    post posts_path, params: { post: { title: "Test" } }
    assert_response :redirect
    follow_redirect!
    assert_response :success
  end
end

Step 6: Write Non-Brittle Assertions

Test Behavior, Not Exact Values:

# BRITTLE - Exact timestamp match
assert_equal "2025-01-15T10:00:00Z", response["created_at"]

# ROBUST - Just verify presence
assert response["created_at"].present?

# BRITTLE - Exact error message
assert_equal "Name can't be blank", record.errors.full_messages.first

# ROBUST - Check for key content
assert_includes record.errors[:name], "can't be blank"

Use Inclusive Assertions:

# BRITTLE - Exact match
assert_equal({ id: 1, name: "Test", email: "test@example.com" }, response)

# ROBUST - Check key attributes only
assert_equal 1, response[:id]
assert_equal "Test", response[:name]
# OR
assert response.slice(:id, :name) == { id: 1, name: "Test" }

Step 7: Handle Multi-Tenancy

For acts_as_tenant projects, always wrap in tenant context:

# WRONG - Missing tenant context
test "admin can view project" do
  assert policy(@admin, @project).show?
end

# CORRECT - Proper tenant scoping
test "admin can view project" do
  with_workspace(@workspace) do
    assert policy(@admin, @project).show?
  end
end

Step 8: Test Permission Flows Correctly

Always test denial BEFORE granting, then allow AFTER:

test "member requires permission to create" do
  with_workspace(@workspace) do
    # 1. Test denial WITHOUT permission
    refute policy(@member, Project).create?
    
    # 2. Grant permission
    set_workspace_permissions(@member, @workspace, :allowed_to_create_projects)
    
    # 3. Test allow WITH permission
    assert policy(@member, Project).create?
  end
end

Quick Reference

Assertion Mapping (RSpec to Minitest)

Rails-Specific Assertions

# Record changes
assert_difference "Post.count", 1 do
  Post.create!(title: "Test")
end

assert_no_difference "Post.count" do
  Post.new.save  # Invalid, doesn't save
end

# Value changes
assert_changes -> { post.reload.title }, from: "Old", to: "New" do
  post.update!(title: "New")
end

# Response assertions
assert_response :success
assert_response :redirect
assert_redirected_to post_path(post)

# DOM assertions
assert_select "h1", "Expected Title"
assert_select ".post", count: 3

# Query assertions
assert_queries_count(2) { User.find(1); User.find(2) }
assert_no_queries { cached_value }

Test File Templates

Model Test:

require "test_helper"

class UserTest < ActiveSupport::TestCase
  setup do
    @user = users(:active_user)
  end

  test "valid fixture" do
    assert @user.valid?
  end

  test "requires email" do
    @user.email = nil
    refute @user.valid?
    assert_includes @user.errors[:email], "can't be blank"
  end

  test "#display_name returns formatted name" do
    @user.name = "John Doe"
    assert_equal "John Doe", @user.display_name
  end
end

Request Test:

require "test_helper"

class PostsRequestTest < ActionDispatch::IntegrationTest
  setup do
    @user = users(:active_user)
    @post = posts(:published_post)
    sign_in @user
  end

  test "GET /posts returns success" do
    get posts_path
    assert_response :success
  end

  test "POST /posts creates record" do
    assert_difference "Post.count", 1 do
      post posts_path, params: { post: { title: "New Post", body: "Content" } }
    end
    assert_redirected_to post_path(Post.last)
  end

  test "POST /posts with invalid data returns error" do
    assert_no_difference "Post.count" do
      post posts_path, params: { post: { title: "" } }
    end
    assert_response :unprocessable_entity
  end
end

Policy Test:

require "test_helper"

class PostPolicyTest < ActiveSupport::TestCase
  include PolicyTestHelpers

  setup do
    @workspace = workspaces(:main_workspace)
    @admin = users(:admin_user)
    @member = users(:member_user)
    @post = posts(:workspace_post)
    Current.user = nil
  end

  test "admin can always edit" do
    with_workspace(@workspace) do
      assert policy(@admin, @post).edit?
    end
  end

  test "member requires permission to edit" do
    with_workspace(@workspace) do
      refute policy(@member, @post).edit?
      
      set_workspace_permissions(@member, @workspace, :allowed_to_edit_posts)
      assert policy(@member, @post).edit?
    end
  end

  test "scope excludes other workspace posts" do
    with_workspace(@other_workspace) do
      scope = PostPolicy::Scope.new(@admin, Post.all).resolve
      refute_includes scope, @post
    end
  end
end

Performance Optimization Checklist

Before submitting tests, verify:

• Using fixtures instead of factories
• Using User.new instead of User.create when DB not needed
• Testing validation errors on in-memory objects
• Minimal fixture data (only what's needed)
• Request tests instead of system tests where possible
• Pagination thresholds configured low for tests
• No unnecessary associations in fixtures
• BCrypt cost set to minimum in test environment
• Logging disabled in test environment
• Using build_stubbed pattern where applicable

Anti-Patterns to Avoid

1. Testing implementation details - Test outcomes, not internal method calls
2. Overly complex setup - If setup is > 10 lines, refactor to fixtures
3. Shared state between tests - Each test should be independent
4. Testing private methods - Only test public interface
5. Brittle assertions - Don't assert on timestamps, exact errors, or order
6. Too many system tests - Reserve for critical user paths only
7. Missing negative tests - Always test what should fail/be denied
8. Factory cascades - Avoid factories that create many associated records

Running Tests

# Run all tests
bin/rails test

# Run specific file
bin/rails test test/models/user_test.rb

# Run specific test by line
bin/rails test test/models/user_test.rb:42

# Run specific test by name
bin/rails test -n "test_requires_email"

# Run directory
bin/rails test test/policies/

# Run with verbose output
bin/rails test -v

# Run in parallel
bin/rails test --parallel

# Run with coverage
COVERAGE=true bin/rails test

# Run and fail fast
bin/rails test --fail-fast

Debugging Tips

# Print response body in request tests
puts response.body

# Print validation errors
pp @record.errors.full_messages

# Use breakpoint (Rails 7+)
debugger

# Check SQL queries
ActiveRecord::Base.logger = Logger.new(STDOUT)

# Inspect fixture data
pp users(:admin_user).attributes