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

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233

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Mobile Development

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

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

01233 GitHub stars

02Post-migration data integrity verification scripts

03Troubleshooting for simulator-only success versus real-device failures

04Validation of SchemaMigrationPlan version order and registration

05Detection of missing relationship inverses and junction table errors

06Systematic diagnostic checklists for identifying SwiftData crash types

Use Cases

01Ensuring data persistence during complex migrations using willMigrate and didMigrate

02Fixing 'Expected only Arrays' errors in many-to-many relationship configurations

03Debugging app launch crashes after modifying SwiftData model schemas

What are Skills?·How to Install

Install with 🐟 Skill.Fish

npx skillfish add charleswiltgen/axiom axiom-swiftdata-migration-diag

For use in Claude.ai and ChatGPT

Download Skill