What causes the 'Expected only Arrays for Relationships' error in SwiftData?

This error typically occurs when many-to-many relationships are missing explicit inverse declarations, preventing SwiftData from creating the necessary junction tables.

Can this skill help with custom migration stages?

Yes, it includes specific patterns for correctly using willMigrate to access old model versions and transform data before the schema change is finalized.

Why does my SwiftData migration work in the simulator but fail on a real device?

The simulator often deletes the database on each rebuild, which can hide schema mismatch issues. Real devices keep persistent databases and will crash immediately if the schema versioning isn't handled correctly.

How do I verify if a SwiftData migration was successful?

This skill provides a verification checklist to compare record counts, spot-check specific records, and confirm that both forward and inverse relationships remain intact after the migration.

SwiftData Migration Diagnostics

Name: SwiftData Migration Diagnostics
Author: CharlesWiltgen

byCharlesWiltgen

•

233

Mobile Development

Diagnoses and resolves SwiftData migration failures, crashes, and data loss issues in Apple platform development.

About

The SwiftData Migration Diagnostics skill provides a systematic framework for troubleshooting complex persistence issues across iOS, iPadOS, watchOS, and tvOS applications. It addresses common pitfalls such as schema version mismatches, missing relationship inverses, and discrepancies between simulator and real-device behavior. By offering structured checklists, decision trees, and battle-tested patterns, it helps developers ensure data integrity and prevent production crashes during complex schema transitions, transforming frustrating 'Incompatible Model' errors into actionable fixes.

Key Features

233 GitHub stars
Post-migration data integrity verification scripts
Troubleshooting for simulator-only success versus real-device failures
Validation of SchemaMigrationPlan version order and registration
Detection of missing relationship inverses and junction table errors
Systematic diagnostic checklists for identifying SwiftData crash types

Use Cases

Ensuring data persistence during complex migrations using willMigrate and didMigrate
Fixing 'Expected only Arrays' errors in many-to-many relationship configurations
Debugging app launch crashes after modifying SwiftData model schemas

About

Key Features

233 GitHub stars
Post-migration data integrity verification scripts
Troubleshooting for simulator-only success versus real-device failures
Validation of SchemaMigrationPlan version order and registration
Detection of missing relationship inverses and junction table errors
Systematic diagnostic checklists for identifying SwiftData crash types

Use Cases

Ensuring data persistence during complex migrations using willMigrate and didMigrate
Fixing 'Expected only Arrays' errors in many-to-many relationship configurations
Debugging app launch crashes after modifying SwiftData model schemas