Database Migration

Safe database schema evolution for SQLite/GRDB/SwiftData. Prevents data loss with additive migrations and testing workflows.

When to use: Adding/modifying database columns, encountering "FOREIGN KEY constraint failed", "no such column", "cannot add NOT NULL column" errors, creating schema migrations for SQLite/GRDB/SwiftData

Key Features

• Safe migration patterns (additive, idempotent, transactional)
• Testing checklist (fresh install + migration paths)
• Common errors and fixes
• GRDB and SwiftData examples
• Multi-layered prevention for 100k+ user apps

Philosophy: Migrations are immutable after shipping. Make them additive, idempotent, and thoroughly tested to prevent data loss.

TDD Tested: Already bulletproof, no changes needed during pressure testing

Example Prompts

These are real questions developers ask that this skill answers:

• "I need to add a column to my live app without losing user data." → Covers safe additive patterns including idempotency checks and avoiding data loss

• "I'm getting 'cannot add NOT NULL column' errors when migrating." → Explains why NOT NULL fails with existing rows and shows the safe pattern (nullable first, backfill later)

• "I need to change a column from text to integer. Can I just ALTER the type?" → Demonstrates the safe pattern: add new column → migrate data → deprecate old (NEVER delete)

• "I'm adding a foreign key relationship. How do I add it without breaking existing data?" → Covers safe patterns: add column → populate data → add index (SQLite limitations explained)

• "Users reported crashes after the last update. I changed a migration in production." → Explains migrations are immutable after shipping and how to create a new migration to fix issues