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Who published Testing?

tomevault-io (@tomevault-io) published this skill. Their other Agent Skills are listed on their SkillMD profile.

Testing by tomevault-io — Agent Skill

Name: Testing
Author: tomevault-io

File contents

Elixir Testing Reference

Quick reference for Elixir testing patterns.

Iron Laws — Never Violate These

ASYNC BY DEFAULT — Use async: true unless tests modify global state
SANDBOX ISOLATION — All database tests use Ecto.Adapters.SQL.Sandbox
MOCK ONLY AT BOUNDARIES — Never mock database, internal modules, or stdlib
BEHAVIOURS AS CONTRACTS — All mocks must implement a defined @callback behaviour
BUILD BY DEFAULT — Use build/2 in factories; insert/2 only when DB needed
NO PROCESS.SLEEP — Use assert_receive with timeout for async operations
VERIFY_ON_EXIT! — Always call in Mox tests setup
FACTORIES MATCH SCHEMA REQUIRED FIELDS — Factory definitions must include all fields that have validate_required in the schema changeset. Missing fields cause cascading test failures

Quick Decisions

Which Test Case?

Testing	Use
Controller/API	`use MyAppWeb.ConnCase`
Context/Schema	`use MyApp.DataCase`
LiveView	`use MyAppWeb.ConnCase` + `import Phoenix.LiveViewTest`
Pure logic	`use ExUnit.Case, async: true`

When to use async: true?

✅ Pure functions, no shared state
✅ Database tests with Sandbox (PostgreSQL)
❌ Tests modifying Application.put_env
❌ Tests using Mox global mode

Mock or not?

✅ Mock: External APIs, email services, file storage
❌ Don't mock: Database, internal modules, stdlib

build() or insert()?

Use build() by default for speed
Use insert() only when you need DB ID, constraints, or persisted associations

Quick Patterns

# Setup chain
setup [:create_user, :authenticate]

# Pattern matching assertion
assert {:ok, %User{name: name}} = create_user(attrs)

# Async message assertion
assert_receive {:user_created, _}, 5000

# Mox setup
setup :verify_on_exit!
expect(MockAPI, :call, fn _ -> {:ok, "data"} end)

# LiveView async
html = render_async(view)  # MUST call for assign_async

Common Anti-patterns

Wrong	Right
`Process.sleep(100)`	`assert_receive {:done, _}, 5000`
`insert(:user)` in factory	`build(:user)` in factory
`async: true` with `set_mox_global()`	`async: false`
Mock internal modules	Test through public API

References

For detailed patterns, see:

${CLAUDE_SKILL_DIR}/references/exunit-patterns.md - Setup, assertions, tags
${CLAUDE_SKILL_DIR}/references/mox-patterns.md - Behaviours, expect/stub, async
${CLAUDE_SKILL_DIR}/references/liveview-testing.md - Forms, async, uploads
${CLAUDE_SKILL_DIR}/references/factory-patterns.md - ExMachina, sequences, traits

Converted and distributed by TomeVault — claim your Tome and manage your conversions.

---name: testing-73description: Elixir testing patterns — ExUnit, Mox, factories, LiveView test helpers. Use when working on *_test.exs, test/support/, factory files, or fixing test failures. Use when this capability is needed.---# Elixir Testing ReferenceQuick reference for Elixir testing patterns.## Iron Laws — Never Violate These1. **ASYNC BY DEFAULT** — Use `async: true` unless tests modify global state2. **SANDBOX ISOLATION** — All database tests use Ecto.Adapters.SQL.Sandbox3. **MOCK ONLY AT BOUNDARIES** — Never mock database, internal modules, or stdlib4. **BEHAVIOURS AS CONTRACTS** — All mocks must implement a defined `@callback` behaviour5. **BUILD BY DEFAULT** — Use `build/2` in factories; `insert/2` only when DB needed6. **NO PROCESS.SLEEP** — Use `assert_receive` with timeout for async operations7. **VERIFY_ON_EXIT!** — Always call in Mox tests setup8. **FACTORIES MATCH SCHEMA REQUIRED FIELDS** — Factory definitions must include all fields that have `validate_required` in the schema changeset. Missing fields cause cascading test failures## Quick Decisions### Which Test Case?| Testing | Use ||---------|-----|| Controller/API | `use MyAppWeb.ConnCase` || Context/Schema | `use MyApp.DataCase` || LiveView | `use MyAppWeb.ConnCase` + `import Phoenix.LiveViewTest` || Pure logic | `use ExUnit.Case, async: true` |### When to use async: true?- ✅ Pure functions, no shared state- ✅ Database tests with Sandbox (PostgreSQL)- ❌ Tests modifying `Application.put_env`- ❌ Tests using Mox global mode### Mock or not?- ✅ Mock: External APIs, email services, file storage- ❌ Don't mock: Database, internal modules, stdlib### build() or insert()?- Use `build()` by default for speed- Use `insert()` only when you need DB ID, constraints, or persisted associations## Quick Patterns```elixir# Setup chainsetup [:create_user, :authenticate]# Pattern matching assertionassert {:ok, %User{name: name}} = create_user(attrs)# Async message assertionassert_receive {:user_created, _}, 5000# Mox setupsetup :verify_on_exit!expect(MockAPI, :call, fn _ -> {:ok, "data"} end)# LiveView asynchtml = render_async(view) # MUST call for assign_async```## Common Anti-patterns| Wrong | Right ||-------|-------|| `Process.sleep(100)` | `assert_receive {:done, _}, 5000` || `insert(:user)` in factory | `build(:user)` in factory || `async: true` with `set_mox_global()` | `async: false` || Mock internal modules | Test through public API |## ReferencesFor detailed patterns, see:- `${CLAUDE_SKILL_DIR}/references/exunit-patterns.md` - Setup, assertions, tags- `${CLAUDE_SKILL_DIR}/references/mox-patterns.md` - Behaviours, expect/stub, async- `${CLAUDE_SKILL_DIR}/references/liveview-testing.md` - Forms, async, uploads- `${CLAUDE_SKILL_DIR}/references/factory-patterns.md` - ExMachina, sequences, traits---> Converted and distributed by [TomeVault](https://tomevault.io/claim/oliver-kriska) — claim your Tome and manage your conversions.

Frequently asked questions

Testing