SwiftData persistence framework patterns for macOS and iOS apps. Covers @Model definitions, @Query, relationships (with delete rules), ModelContainer/ModelContext configuration, class inheritance with @Model subclasses, type-based predicates (#Predicate with is/as? casting), polymorphic relationships, @Attribute options (.preserveValueOnDeletion), schema migrations, and import/export. Use when implementing data persistence, defining models, querying data, or working with SwiftData. Prevents common LLM errors like generating Core Data syntax, inventing nonexistent SwiftData APIs, or misusing @Query.