Elixir Testing Reference

Quick reference for Elixir testing patterns.

Iron Laws — Never Violate These

1. ASYNC BY DEFAULT — Use async: true unless tests modify global state
2. SANDBOX ISOLATION — All database tests use Ecto.Adapters.SQL.Sandbox
3. MOCK ONLY AT BOUNDARIES — Never mock database, internal modules, or stdlib
4. BEHAVIOURS AS CONTRACTS — All mocks must implement a defined @callback behaviour
5. BUILD BY DEFAULT — Use build/2 in factories; insert/2 only when DB needed
6. NO PROCESS.SLEEP — Use assert_receive with timeout for async operations
7. VERIFY_ON_EXIT! — Always call in Mox tests setup
8. 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?

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

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