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.
2. For data+schema, create custom migration first:
   • bun run db:generate -- --custom --name <descriptive_name>
   • Add idempotent backfill SQL.
3. Generate schema migration second:
   • bun run db:generate
4. Verify migration ordering in drizzle/meta/_journal.json:
   • backfill migration index must be lower than constraint-tightening migration index.
5. Verify generated SQL and snapshots:
   • backfill migration contains only intended data change.
   • schema migration contains constraint/default/type changes.
6. Run full backend verification:
   • bun run lint && bun run typecheck && bun run test
7. 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.