inheritance_skill

Overview

This skill explains class inheritance patterns for SwiftData models, focusing on when to choose inheritance vs enums or protocols. It shows how to annotate base and subclass models, configure ModelContainer, and query across hierarchies while avoiding common pitfalls. Practical guidance covers relationships, type-based filtering, and migration considerations.

How this skill works

The skill inspects model design choices and provides concrete code patterns: apply @Model to the base class and every subclass, place shared persistent properties on the base, and register only the base with ModelContainer. It describes querying the base class to return all subclasses, filtering with #Predicate using type checks, and safely casting to access subclass fields in UI and business logic. It also highlights mistakes that cause lost persisted data or migration headaches.

When to use it

• You have a clear IS‑A relationship where subclasses add significant stored properties or behavior
• You need both deep queries (all base instances) and shallow queries (only one subtype)
• You need polymorphic relationships that hold mixed subtype instances
• You are migrating a Core Data inheritance hierarchy to SwiftData
• You want a shallow hierarchy (base + one tier) and manageable migrations

Best practices

• Apply @Model to the base class and every subclass so subclass properties persist
• Keep the hierarchy shallow: base class plus one level of subclasses only
• Call super.init(...) from every subclass initializer with all required base properties
• Register the base class in ModelContainer; SwiftData auto-discovers subclasses
• Prefer an enum on the base model when differences are only a type tag or one/two optional fields
• Use #Predicate and is/as? checks for type-based filtering and safe casting in views

Example use cases

• Trip model where BusinessTrip and PersonalTrip share identity and many fields, but add distinct properties
• A polymorphic relationship: Accommodation.trip can point to any Trip subtype at runtime
• UI with segmented filter switching between all trips, business trips, and personal trips
• Migrating a two-level Core Data inheritance scheme to SwiftData with minimal schema changes
• Choosing enum on Trip for small variant differences to avoid migration complexity