Model Domain

Quick Reference

DDD concept	Ruby default	Avoid by default	Typical home
Entity	Plain class when persisted identity matters	Extra wrapper object with no added meaning	`models/` or project namespace
Value object	PORO — immutable, equality by value	Shoving logic into helpers or primitives	`models/` or near the domain
Aggregate root	The class that guards invariants and is the single entry point	Splitting invariants across multiple models/classes	`models/` or project namespace
Domain service	PORO for behavior spanning multiple entities	Arbitrary model chosen just to hold code	`services/`
Application service	Orchestrator for one use case	Fat controllers, callback chains, or leaking rules	`services/`
Repository	Only when a real persistence boundary exists beyond simple data access	Repositories for every query	`repositories/`
Domain event	Explicit object when multiple downstream consumers justify it	Callback-driven hidden side effects	`events/` or project namespace