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?