Rails Testing

Community

marckohlbrugge

rails-testing

Apply Rails testing standards with Minitest, fixtures, and pragmatic coverage boundaries. Use when creating tests, reviewing test quality, or improving flaky and slow Rails test suites.

Overview

Publisher	marckohlbrugge
Repository	`37signals-skills`
Skill name	`rails-testing`
Stars	724
Forks	56
Bundled files	Instructions only
Links	Skill source Repository

Markdown instructions
A SKILL.md file the model loads on demand, so it only costs tokens when a request actually matches.
Works with any LLM
AI skills are plain Markdown, not provider-specific code, so this works with GPT, Claude, Gemini, Grok, or a local model.
Self-contained
Everything the model needs lives in the instructions — no extra files to sync.
Open source
Published by marckohlbrugge on GitHub. Read the source before you install it.

Installation

Install the Rails Testing AI skill in TypingMind to use it with any LLM, or drop it into another agent that reads SKILL.md.

Install in TypingMind

TypingMind installs a skill straight from its GitHub folder — it reads SKILL.md, bundles the resource files, and stores the result locally.

Open the app and go to Plugins → Skills.
Choose "Install from GitHub".
Paste the skill folder URL below and confirm.
https://github.com/marckohlbrugge/37signals-skills/tree/main/skills/rails-testing
Enable the skill in any chat where you want it available.

Plugins → Skills → Add skill → From GitHub URL, then paste the folder URL and press Continue.

Install in another agent

Any agent that reads the Agent Skills format can use this skill — copy the folder into that agent's skills directory.

Claude Code — .claude/skills

git clone --depth 1 https://github.com/marckohlbrugge/37signals-skills.git /tmp/37signals-skills
mkdir -p .claude/skills
cp -r /tmp/37signals-skills/skills/rails-testing .claude/skills/rails-testing

Restart Claude Code after copying so it picks up the new skill.

Use it in TypingMind

Enable Rails Testing in any TypingMind chat and the model takes it from there. Its name and description sit in the system prompt, and the moment a request matches, the model loads the full instructions itself — you never invoke it by hand, and it costs no tokens until it is actually used.

The model loads Rails Testing on its own as soon as a request matches it.

Works with any AI model

AI skills are plain Markdown instructions rather than provider-specific code, so Rails Testing is not tied to the model it was written for. Install it once in TypingMind and use it with GPT-5, Claude, Gemini, Grok, DeepSeek, Mistral, Llama, or a local model you run yourself — all on your own API keys.

Loaded only when it is needed
The system prompt carries just the name and description. The instructions are fetched on the first matching request, so an idle skill costs nothing.
Switch models mid-chat
Because the skill is instructions rather than code, changing model does not break it — the next model reads the same SKILL.md.

Skill instructions

This is the SKILL.md content the model loads. Read it before installing — a skill is instructions your model will follow.

Rails Testing

Use for test-writing and test-review tasks. Patterns from Campfire and Fizzy test suites.

Defaults

Minitest + fixtures. No RSpec, no FactoryBot.
Test behavior, not implementation details.
Keep tests deterministic and fast; parallelize(workers: :number_of_processors).
Tests ship in the same commit/PR as the feature — not before, not later. Security fixes always include a regression test.
Never add production complexity for testability (no test-induced design damage).

Coverage Budget (where 37signals actually spends)

Heavy: model tests (domain invariants, concerns) and controller/integration tests (full request cycle, auth, formats).
Light: a few system tests for the critical happy paths (one smoke test can cover signup→use); job tests only for jobs with real logic.
None: view tests, JS/Stimulus unit tests, exhaustive channel tests. UI behavior is covered indirectly by system tests.
Don't duplicate the same behavior assertion at multiple layers.

Fixtures

Express relationships by label, not ID; use ERB for relative timestamps (created_at: <%= 1.hour.ago %>) and shared computed values (one bcrypt digest reused).
Mirror app/models structure in test/models: app/models/card/closeable.rb ↔ test/models/card/closeable_test.rb; shared concerns under test/models/concerns/.
UUID PKs break fixture ordering: generate deterministic, label-derived UUIDv7s in fixtures so .first/.last are stable and runtime records are always newer.
Build rich-content fixtures with production code (ActionText::Attachment.from_attachable(user).to_html), not hand-written markup.

Good Practices

Use system/integration tests for user workflows; model tests for domain invariants.
travel_to for time-based logic.
Mock/stub only at boundaries (external APIs, network, time, SecureRandom); use VCR for external HTTP — auto-name cassettes from class+test name, normalize timestamps in matching.
Test async side effects from model tests with perform_enqueued_jobs(only: SpecificJob) and assert_enqueued_with — not by unit-testing trivial job classes.
Correlated count changes in one assertion: assert_difference({ -> { card.assignees.count } => -1, -> { Event.count } => +1 }).
Test both response formats where controllers serve them: as: :turbo_stream (assert stream targets) and as: :json (status, Location header, body).
Turbo/broadcast assertions by layer: assert_turbo_stream_broadcasts in model tests, assert_turbo_stream action:, target: in controller tests, assert_no_turbo_stream_broadcasts for negatives.
Authorization tests assert the negative space: cross-tenant/role access returns 403/404, not just that allowed access works.
Multi-tenant suites: set Current.account (and Current.session when behavior depends on the actor) in setup; integration/system tests set default_url_options[:script_name]; provide an untenanted { } helper for auth routes. Clear Current in teardown.
Test middleware in isolation with Rack::MockRequest.
System tests: using_session("Kevin") for multi-user scenarios; wait for cable connection before asserting realtime; auth via a fast session-transfer helper, keeping the full login flow to one smoke test.
Suites with non-transactional side effects (FTS tables) opt out per-helper: self.use_transactional_tests = false + explicit cleanup.
Reset shared global state per test in parallel suites (thread pools, ActionCable.server.pubsub, Current).

Red Flags

Adding production complexity only for testability.
Over-mocking internal app code.
Duplicate tests for the same behavior at multiple layers.
Slow suites caused by unnecessary setup in each test (that's what fixtures are for).
Unit tests for one-line job classes or trivial delegations.
Hand-rolled HTML strings where production renderers/helpers would stay in sync automatically.
Time-dependent assertions without travel_to.

Frequently asked questions

What does the Rails Testing AI skill do?

Apply Rails testing standards with Minitest, fixtures, and pragmatic coverage boundaries. Use when creating tests, reviewing test quality, or improving flaky and slow Rails test suites.

Why use Rails Testing on TypingMind?

Because you install it once and use it with any model. Rails Testing is plain Markdown rather than provider-specific code, so the same skill runs on GPT-5, Claude, Gemini, Grok, or a local model — and you can switch model mid-chat without it breaking. TypingMind runs on your own API keys, so you pay providers directly instead of a per-seat subscription, and your skills and chats stay in your own storage.

How do I install Rails Testing in TypingMind?

Open Plugins → Skills → Install from GitHub in TypingMind and paste https://github.com/marckohlbrugge/37signals-skills/tree/main/skills/rails-testing. TypingMind reads its SKILL.md and installs it as a skill you can enable per chat.

Which AI models can use Rails Testing?

Any model you connect in TypingMind. AI skills are plain Markdown instructions rather than provider-specific code, so GPT, Claude, Gemini, Grok, and local models can all load this skill when a request matches it.

How many AI models can I use with Rails Testing?

As many as you like. As long as a model supports skills, you can use Rails Testing with it — GPT, Claude, Gemini, Grok, DeepSeek, Mistral, Llama and more — all on TypingMind with your own API keys.

Is the Rails Testing AI skill free?

It is published on GitHub by marckohlbrugge. Check the repository for licensing terms. You only pay your own AI provider for the tokens you use.

What are AI skills?

An AI skill is a reusable instruction bundle that teaches an AI model how to do one specific task. It follows the open Agent Skills format: a SKILL.md file with a name and description, plus any scripts, templates or reference files the model may need. The model reads the instructions only when your request matches the skill, so an installed skill costs nothing until it is used.

How are AI skills different from plugins or MCP servers?

A plugin or MCP server gives a model new tools to call — code that runs somewhere and returns a result. An AI skill gives the model knowledge and process instead: how to approach a task, which steps to follow, what good output looks like. Skills are plain Markdown, so they need no server, no API key and no runtime, and they work with any model.

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Overview

Installation

Install in TypingMind

Install in another agent

Use it in TypingMind

Works with any AI model

Skill instructions

Rails Testing

Defaults

Coverage Budget (where 37signals actually spends)

Fixtures

Good Practices

Red Flags

Frequently asked questions

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