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


model-
persist-
query-
crud-
sync-
rel-
state-
schema-
preview-

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

NPX Install

Tags

SKILL.md Content