leonanpereirapinto/ai-utils/drizzle-safe-migrations

Drizzle Safe Migrations

Overview

Use this skill to run database migrations in a way that is auditable, deployment-safe, and consistent with Drizzle's migration model.

Core Rules

• Always generate schema migrations with the project script (bun run db:generate).
• Never hand-edit generated schema migration files.
• Generate data backfills as custom migrations (bun run db:generate -- --custom --name <name>) and edit only that custom SQL file.
• Apply data normalization before tightening constraints.
• Keep one-off data fixes in migration history, not as hidden runtime logic, unless an emergency hotfix requires temporary mitigation.

Workflow

1. Classify the change:

• schema-only: only column/table/index/default changes.
• data+schema: old rows must be transformed before new constraints/defaults.

1. For data+schema, create custom migration first:

• bun run db:generate -- --custom --name <descriptive_name>
• Add idempotent backfill SQL.

1. Generate schema migration second:

• bun run db:generate

1. Verify migration ordering in drizzle/meta/_journal.json:

• backfill migration index must be lower than constraint-tightening migration index.

1. Verify generated SQL and snapshots:

• backfill migration contains only intended data change.
• schema migration contains constraint/default/type changes.

1. Run full backend verification:

• bun run lint && bun run typecheck && bun run test

1. Document deployment notes:

• expected data transformations,
• lock-risk areas,
• rollback strategy.

Backfill Requirements

• Use restrictive WHERE clauses.
• Prefer idempotent updates (UPDATE ... WHERE status = 'legacy_value').
• Do not mix unrelated DDL/DML in the same migration.
• Keep SQL explicit and minimal.

Constraint Tightening Pattern

When removing allowed values (enum/check):

1. Backfill existing rows to valid target value.
2. Update default to new value.
3. Tighten check/enum constraint.

For large tables or strict uptime targets, use staged PostgreSQL patterns (NOT VALID + VALIDATE CONSTRAINT) where applicable.

Anti-Patterns

• Hand-editing generated schema migration files.
• Tightening constraints before backfilling existing data.
• Hiding one-time migration logic in app startup code without migration artifacts.
• Running migrations without validating order in the Drizzle journal.

Reference

• See references/production-playbook.md for command templates and review checklists.