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minitest-coder Write Minitest tests for Ruby and Rails applications. Use when creating test files, writing test cases, or testing new features. Not for RSpec — use rspec-coder instead. Covers both traditional and spec styles, fixtures, mocking, and Rails integration testing patterns.
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name minitest-coder description Write Minitest tests for Ruby and Rails applications. Use when creating test files, writing test cases, or testing new features. Not for RSpec — use rspec-coder instead. Covers both traditional and spec styles, fixtures, mocking, and Rails integration testing patterns. allowed-tools Read Write Edit MultiEdit Grep Glob Bash WebSearch
Minitest Coder
Core Philosophy
AAA Pattern : Arrange-Act-Assert structure for clarityBehavior over Implementation : Test what code does, not howIsolation : Tests should be independentDescriptive Names : Clear test descriptionsCoverage : Test happy paths AND edge casesFast Tests : Minimize database operationsFixtures : Use fixtures for test data
Minitest Styles
Style Best For Syntax Traditional Simple unit tests test "description"Spec Complex scenarios with contexts describe/it with let/subject
Traditional Style
class UserTest < ActiveSupport::TestCase
test "validates presence of name"
do
User
: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 teststest/services/ - Service object teststest/integration/ - Full-stack teststest/mailers/ - Mailer teststest/jobs/ - Background job teststest/fixtures/ - Test datatest/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 Pattern Use Case subject { ... }Method under test let(:name) { ... }Lazy-evaluated data describe "context"Group related tests (max 3 levels) before { ... }Complex setup
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
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 Method Purpose Object.stub :method, valueStub return value Minitest::Mock.newVerify method calls
test "processes payment" do
PaymentGateway .stub :charge , true do
processor = OrderProcessor .new(order)
assert processor.process
end
end
Assertions Quick Reference
Basic Assertions
assert user.valid?
assert_not user.admin?
assert_equal "Alice" , user.name
assert_nil user.deleted_at
assert_includes users, admin_user
assert_empty order.items
assert_raises ActiveRecord : :RecordInvalid do
user.save!
end
Rails Assertions
assert_changes -> { user.reload.status }, to: "active" do
user.activate!
end
assert_difference "User.count" , 1 do
User .create(name: "Charlie" )
end
assert_response :success
assert_redirected_to user_path(user)
AAA Pattern test "processes refund" do
order = orders(:completed_order )
original_balance = order.user.account_balance
result = order.process_refund
assert result.success?
assert_equal "refunded" , order.reload.status
end
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 Type Test For Models Validations, associations, scopes, callbacks, methods Services Happy path, sad path, edge cases, external integrations Controllers Status codes, redirects, parameter handling Jobs Execution, retry logic, error handling
Coverage Example class UserTest < ActiveSupport::TestCase
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
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 Pattern Problem Solved Current.account fixtures Multi-tenant URL isolation Assertion-validating helpers Early failure with clear messages Deterministic UUIDs Predictable fixture ordering VCR timestamp filtering Reusable API cassettes Thread-based concurrency Race condition detection Adapter-aware helpers SQLite/MySQL compatibility
Anti-Patterns Anti-Pattern Why Bad require 'test_helper'Auto-imported >3 nesting levels Unreadable output @ivars instead of letState leakage Missing subject Repetitive code assert x.include?(y)Use assert_includes Testing private methods Implementation coupling Not using fixtures Slow tests
Best Practices Checklist
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.
