Models & Domain Objects

• No service objects. All domain classes live in app/models/ with namespaces, never app/services/.
• Name classes after domain nouns, not actions. No *Service, *Manager, *Handler suffixes.
• Use ActiveModel::Model for POROs that need validation or form integration.
• Replace .call / .perform with domain verbs: #save, #complete, #submit, #deliver.
• Look to identify domain models that can be extracted when an existing model exceeds: 200 lines, 15 public methods, or 7 private methods.
• Callbacks only for data integrity (normalise fields, set defaults). Never for emails, payments, or external systems.
• Prefer composition over inheritance. Extract behaviour into small, focused objects.
• Avoid feature envy, long parameter lists (max 3 args), case statements on type, and mixin abuse.