rails-testing-patterns

name	rails-testing-patterns
description	Rails/Minitest test-type selection (integration over system tests), XSS dual-verification, assert_select scoping, AJAX contract testing, and object-staleness (.reload) pitfalls. Minitest/ActiveSupport::TestCase examples — adapt for RSpec.
license	MIT
metadata	{"version":"2.1.0","hermes":{"tags":["Rails","Testing","Patterns","Quality"],"related_skills":["rails-cache-performance","test-plan-methodology"]}}

Rails Testing Pattern Selection

Test Type Decision

Question	Test Type
Requires real browser (modals, JS-heavy UI)?	System test (last resort — markedly slower: browser boot + JS)
Controller logic, routing, template rendering, security?	Integration test
Frontend controller / DOM manipulation?	JavaScript test (Jest/Mocha)
Business logic, validations, associations?	Model test

Default to integration tests. System tests only for genuine browser interaction.

XSS Dual-Verification

Simple absence checks fail when content is HTML-escaped. Always verify both sides:

test 'escapes user input in HTML' do
  item = create(:item, name: '<img onerror="alert(1)">')
  get item_path(item)

  assert_includes response.body, '&lt;img'                    # entities present
  assert_not_includes response.body, '<img onerror="alert'     # raw markup absent
end

assert_select Patterns

Use minimum: for variable collections, count: only for fixed structures. Always scope utility classes to a container:

assert_select '.product-card', minimum: 1          # variable collection
assert_select 'form.product-form', count: 1        # known singleton

assert_select '#product-details' do                 # scoped — avoids false matches
  assert_select '.text-center', minimum: 1
end

Contract Testing for AJAX

Test the AJAX endpoint directly — AJAX content is absent from the initial page render. Assert contract shape (keys, types), not specific values:

test 'returns expected JSON structure' do
  get api_products_path, as: :json
  json = JSON.parse(response.body)

  assert json.key?('products')
  assert_not_empty json['products']            # guard before .first — nil.key? would crash
  assert json['products'].first.key?('id')
end

(assert_select lives in the rails-controller-testing gem since Rails 5 — add it if the helper is undefined.)

Staleness Pitfalls

Three common variants of the same root cause: the Ruby object doesn't reflect the current DB state. Fix: call .reload before assertions or path helpers.

Scenario	What goes stale	Fix
Create child via parent_id:, then read parent.children	Association cache	parent.reload
Update a FriendlyId model, then use product_path(product)	Slug attribute	product.reload
Factory creates its own association, ignoring your override	Implicit association	Pass association explicitly and verify with .reload

# All three reduce to:
parent = create(:parent)
create(:child, parent_id: parent.id)
parent.reload                          # without this, parent.children is []

Naming Convention

Prefix integration test classes with the type suffix to avoid collisions across directories: FooIntegrationTest < ActionDispatch::IntegrationTest, not FooTest.