Swiftdata Architecture

• Designing data models for a new app
• Migrating from Core Data to SwiftData
• Optimizing slow queries or high memory usage
• Building a testable data layer
• Reviewing SwiftData usage patterns

Quick Decision Guide

Common Pitfalls

1. Missing Unique Constraints

// Wrong - duplicate entries on re-import
@Model class Contact {
    var email: String
    var name: String
}

// Right - prevent duplicates
@Model class Contact {
    #Unique<Contact>([\.email])
    var email: String
    var name: String
}

2. Fetching Too Much Data

// Wrong - loads all properties of all records
let descriptor = FetchDescriptor<Document>()
let allDocs = try modelContext.fetch(descriptor)

// Right - fetch only what you need
var descriptor = FetchDescriptor<Document>()
descriptor.propertiesToFetch = [\.title, \.createdAt]
descriptor.fetchLimit = 50
let docs = try modelContext.fetch(descriptor)

3. Modifying Models on Wrong Context

// Wrong - model from main context modified on background
let doc = documents.first!
Task.detached {
    doc.title = "Updated"  // Thread safety violation!
}

// Right - use background ModelContext
let container = modelContext.container
Task.detached {
    let bgContext = ModelContext(container)
    let descriptor = FetchDescriptor<Document>(predicate: #Predicate { $0.id == docID })
    if let doc = try bgContext.fetch(descriptor).first {
        doc.title = "Updated"
        try bgContext.save()
    }
}

How to Conduct Reviews

Step 1: Understand the Data Model

• What entities exist and how do they relate?
• What's the expected data volume?
• What are the primary query patterns?

Step 2: Review Against Module Guidelines

• Schema design (see schema-design.md)
• Query patterns (see query-patterns.md)
• Repository pattern (see repository-pattern.md)
• Performance (see performance.md)

Step 3: Provide Structured Feedback

For each issue found:

1. Issue: Describe the data layer problem
2. Impact: Data corruption, performance, memory, testability
3. Fix: Correct implementation with code
4. Migration: Note if schema changes require migration

Module References

Load these modules as needed:

1. Schema Design: schema-design.md

   • @Model design and attributes
   • Relationships and cascade rules
   • Unique constraints and indexes

2. Query Patterns: query-patterns.md

   • @Query in SwiftUI views
   • FetchDescriptor for services
   • Predicates, sorting, pagination

3. Repository Pattern: repository-pattern.md

   • Protocol-based abstraction
   • Dependency injection
   • Testing with mock repositories

4. Performance: performance.md

   • Batch operations
   • Background contexts
   • Memory optimization

Response Guidelines

• Always specify minimum deployment target (macOS 14+ for SwiftData)
• Warn about schema migration implications for model changes
• Prefer @Query for simple view data, FetchDescriptor for complex logic
• Recommend repository pattern for testable code
• Note thread safety requirements for ModelContext