SwiftData Best Practices — Modular MVVM-C Data Layer

Comprehensive data modeling, persistence, sync architecture, and error handling guide for SwiftData aligned with the clinic modular MVVM-C stack.

Architecture Alignment

This skill enforces the same modular architecture mandated by swift-ui-architect:

┌───────────────────────────────────────────────────────────────┐
│ Feature modules: View + ViewModel, no SwiftData imports       │
├───────────────────────────────────────────────────────────────┤
│ Domain: models + repository/coordinator/error protocols        │
├───────────────────────────────────────────────────────────────┤
│ Data: @Model entities, SwiftData stores, repository impls,     │
│ remote clients, retry executor, sync queue, conflict handling  │
└───────────────────────────────────────────────────────────────┘

Key principle: SwiftData types (@Model, ModelContext, @Query, FetchDescriptor) live in Data-only implementation code. Feature Views/ViewModels work with Domain types and protocol dependencies.

Clinic Architecture Contract (iOS 26 / Swift 6.2)

All guidance in this skill assumes the clinic modular MVVM-C architecture:

Feature modules import Domain + DesignSystem only (never Data, never sibling features)
App target is the convergence point and owns DependencyContainer, concrete coordinators, and Route Shell wiring
Domain stays pure Swift and defines models plus repository, *Coordinating, ErrorRouting, and AppError contracts
Data owns SwiftData/network/sync/retry/background I/O and implements Domain protocols
Read/write flow defaults to stale-while-revalidate reads and optimistic queued writes
ViewModels call repository protocols directly (no default use-case/interactor layer)

When to Apply

Reference these guidelines when:

Defining @Model entity classes and mapping them to domain structs
Setting up ModelContainer and ModelContext in the Data layer
Implementing repository protocols backed by SwiftData
Writing stale-while-revalidate repository reads (AsyncStream)
Implementing optimistic writes plus queued sync operations
Configuring entity relationships (one-to-many, inverse)
Fetching from APIs and persisting to SwiftData via sync coordinators
Handling save failures, corrupt stores, and migration errors
Routing AppError traits to centralized error UI infrastructure
Building preview infrastructure with sample data
Planning schema migrations for app updates

Workflow

Use this workflow when designing or refactoring a SwiftData-backed feature:

Domain design: define domain structs (Trip, Friend) with validation/computed rules (see model-domain-mapping, state-business-logic-placement)
Entity design: define @Model entity classes with mapping methods (see model-*, model-domain-mapping)
Repository protocol: define in Domain layer, implement with SwiftData in Data layer (see persist-repository-wrapper)
Container wiring: configure ModelContainer once at the app boundary with error recovery (see persist-container-setup, persist-container-error-recovery)
Dependency injection: inject repository protocols via @Environment (see state-dependency-injection)
ViewModel: create @Observable ViewModel that delegates directly to repository protocols (see state-query-vs-viewmodel)
CRUD flows: route all insert/delete/update through ViewModel -> Repository (see crud-*)
Sync architecture: queue writes, execute via sync coordinator with retry policy (see sync-*)
Relationships: model to-many relationships as arrays; define delete rules (see rel-*)
Previews: create in-memory containers and sample data for fast iteration (see preview-*)
Schema evolution: plan migrations with versioned schemas (see schema-*)

Troubleshooting

Data not persisting -> persist-model-macro, persist-container-setup, persist-autosave, schema-configuration
List not updating after background import -> query-background-refresh, persist-model-actor
List not updating (same-context) -> query-property-wrapper, state-wrapper-views
Duplicates from API sync -> schema-unique-attributes, sync-conflict-resolution
App crashes on launch after model change -> schema-migration-recovery, persist-container-error-recovery
Save failures silently losing data -> crud-save-error-handling
Stale data from network -> sync-offline-first, sync-fetch-persist
Widget/extension can't see data -> persist-app-group, schema-configuration
Choosing architecture pattern for data views -> state-query-vs-viewmodel, persist-repository-wrapper

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	Data Modeling	CRITICAL	`model-`
2	Persistence Setup	CRITICAL	`persist-`
3	Querying & Filtering	HIGH	`query-`
4	CRUD Operations	HIGH	`crud-`
5	Sync & Networking	HIGH	`sync-`
6	Relationships	MEDIUM-HIGH	`rel-`
7	SwiftUI State Flow	MEDIUM-HIGH	`state-`
8	Schema & Migration	MEDIUM-HIGH	`schema-`
9	Sample Data & Previews	MEDIUM	`preview-`

Quick Reference

1. Data Modeling (CRITICAL)

model-domain-mapping - Map @Model entities to domain structs across Domain/Data boundaries
model-custom-types - Use custom types over parallel arrays
model-class-for-persistence - Use classes for SwiftData entity types
model-identifiable - Conform entities to Identifiable with UUID
model-initializer - Provide custom initializers for entity classes
model-computed-properties - Use computed properties for derived data
model-defaults - Provide sensible default values for entity properties
model-transient - Mark non-persistent properties with @Transient
model-external-storage - Use external storage for large binary data

2. Persistence Setup (CRITICAL)

persist-repository-wrapper - Wrap SwiftData behind Domain repository protocols
persist-model-macro - Apply @Model macro to all persistent types
persist-container-setup - Configure ModelContainer at the App level
persist-container-error-recovery - Handle ModelContainer creation failure with store recovery
persist-context-environment - Access ModelContext via @Environment (Data layer)
persist-autosave - Enable autosave for manually created contexts
persist-enumerate-batch - Use ModelContext.enumerate for large traversals
persist-in-memory-config - Use in-memory configuration for tests and previews
persist-app-group - Use App Groups for shared data storage
persist-model-actor - Use @ModelActor for background SwiftData work
persist-identifier-transfer - Pass PersistentIdentifier across actors

3. Querying & Filtering (HIGH)

query-property-wrapper - Use @Query for declarative data fetching (Data layer)
query-background-refresh - Force view refresh after background context inserts
query-sort-descriptors - Apply sort descriptors to @Query
query-predicates - Use #Predicate for type-safe filtering
query-dynamic-init - Use custom view initializers for dynamic queries
query-fetch-descriptor - Use FetchDescriptor outside SwiftUI views
query-fetch-tuning - Tune FetchDescriptor paging and pending-change behavior
query-localized-search - Use localizedStandardContains for search
query-expression - Use #Expression for reusable predicate components (iOS 18+)

4. CRUD Operations (HIGH)

crud-insert-context - Insert models via repository implementations
crud-delete-indexset - Delete via repository with IndexSet from onDelete
crud-sheet-creation - Use sheets for focused data creation via ViewModel
crud-cancel-delete - Avoid orphaned records by persisting only on save
crud-undo-cancel - Enable undo and use it to cancel edits
crud-edit-button - Provide EditButton for list management
crud-dismiss-save - Dismiss modal after ViewModel save completes
crud-save-error-handling - Handle repository save failures with user feedback

5. Sync & Networking (HIGH)

sync-fetch-persist - Use injected sync services to fetch and persist API data
sync-offline-first - Design offline-first architecture with repository reads and background sync
sync-conflict-resolution - Implement conflict resolution for bidirectional sync

6. Relationships (MEDIUM-HIGH)

rel-optional-single - Use optionals for optional relationships
rel-array-many - Use arrays for one-to-many relationships
rel-inverse-auto - Rely on SwiftData automatic inverse maintenance
rel-delete-rules - Configure cascade delete rules for owned relationships
rel-explicit-sort - Sort relationship arrays explicitly

7. SwiftUI State Flow (MEDIUM-HIGH)

state-query-vs-viewmodel - Route all data access through @Observable ViewModels
state-business-logic-placement - Place business logic in domain value types and repository-backed ViewModels
state-dependency-injection - Inject repository protocols via @Environment
state-bindable - Use @Bindable for two-way model binding
state-local-state - Use @State for view-local transient data
state-wrapper-views - Extract wrapper views for dynamic query state

8. Schema & Migration (MEDIUM-HIGH)

schema-define-all-types - Define schema with all model types
schema-unique-attributes - Use @Attribute(.unique) for natural keys
schema-unique-macro - Use #Unique for compound uniqueness (iOS 18+)
schema-index - Use #Index for hot predicates and sorts (iOS 18+)
schema-migration-plan - Plan migrations before changing models
schema-migration-recovery - Plan migration recovery for schema changes
schema-configuration - Customize storage with ModelConfiguration

9. Sample Data & Previews (MEDIUM)

preview-sample-singleton - Create a SampleData singleton for previews
preview-in-memory - Use in-memory containers for preview isolation
preview-static-data - Define static sample data on model types
preview-main-actor - Annotate SampleData with @MainActor

How to Use

Read individual reference files for detailed explanations and code examples:

Section definitions - Category structure and impact levels
Rule template - Template for adding new rules

Reference Files

File	Description
references/_sections.md	Category definitions and ordering
assets/templates/_template.md	Template for new rules
metadata.json	Version and reference information

swift-data