# Prisma Migrations

> Manage database schema evolution with prisma migrate dev/deploy/reset and migration history

## When to Use

- Applying schema changes to the database during development
- Deploying migrations to staging or production environments
- Baselining an existing database for Prisma Migrate adoption
- Resolving migration history conflicts or drift

## Instructions

1. **Development workflow** — after editing `schema.prisma`, run:

```bash
npx prisma migrate dev --name describe_the_change
```

This generates a migration SQL file, applies it, and regenerates Prisma Client.

2. **Production deployment** — never use `migrate dev` in production. Use:

```bash
npx prisma migrate deploy
```

This applies all pending migrations without generating new ones.

3. **Reset the database** (development only) — drops the database, recreates it, applies all migrations, and runs the seed script:

```bash
npx prisma migrate reset
```

4. **Baseline an existing database** — when adopting Prisma Migrate on a database that already has tables:

```bash
mkdir -p prisma/migrations/0_init
npx prisma migrate diff --from-empty --to-schema-datamodel prisma/schema.prisma --script > prisma/migrations/0_init/migration.sql
npx prisma migrate resolve --applied 0_init
```

5. **Check for drift** between the schema and the database:

```bash
npx prisma migrate diff --from-schema-datamodel prisma/schema.prisma --to-schema-datasource prisma/schema.prisma
```

6. **Customize generated SQL** — after `migrate dev` creates a migration file, edit it before committing. Prisma will not overwrite your edits on subsequent runs.

7. **Handle failed migrations** — if a migration partially applied:

```bash
# Fix the database state manually, then mark as applied:
npx prisma migrate resolve --applied MIGRATION_NAME
# Or mark as rolled back:
npx prisma migrate resolve --rolled-back MIGRATION_NAME
```

8. **Shadow database** — `migrate dev` requires a shadow database to detect drift. Configure it explicitly if your provider does not allow automatic creation:

```prisma
datasource db {
  provider          = "postgresql"
  url               = env("DATABASE_URL")
  shadowDatabaseUrl = env("SHADOW_DATABASE_URL")
}
```

9. Always commit migration files (`prisma/migrations/`) to version control. They are the source of truth for production.

10. Never delete or reorder migration directories once they have been applied to any environment.

## Details

Prisma Migrate uses a migrations table (`_prisma_migrations`) to track which migrations have been applied. Each migration is a directory containing a `migration.sql` file and is applied in lexicographic order.

**Migration history vs schema state:** The migration history is the authoritative record. The schema file describes the desired end state. If these diverge (drift), `migrate dev` will detect it via the shadow database.

**Shadow database mechanics:** During `migrate dev`, Prisma creates a temporary shadow database, applies all existing migrations to it, computes the diff against your schema, and generates the new migration. The shadow database is dropped afterward. This means you need `CREATE DATABASE` privileges in development.

**Data migrations:** Prisma generates only DDL (schema changes). If you need DML (data changes — backfills, column value transforms), add raw SQL to the generated migration file before committing.

**Trade-offs:**

- `migrate dev` is safe and reversible (via `reset`), but slow on large schemas due to shadow database overhead
- `migrate deploy` is fast but irreversible — there is no built-in rollback. Write reverse migrations manually if needed
- `db push` skips migration history entirely — use it only for rapid prototyping, never in a shared environment

**Common mistakes:**

- Running `migrate dev` in CI — it generates files and requires a shadow database. Use `migrate deploy` instead
- Deleting migration files to "clean up" — this breaks the migration history for any database that already applied them
- Not testing migrations on a copy of production data before deploying — column type changes or NOT NULL additions can fail on existing data

## Source

https://prisma.io/docs/orm/prisma-migrate

## Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

## Harness Integration

- **Type:** knowledge — this skill is a reference document, not a procedural workflow.
- **No tools or state** — consumed as context by other skills and agents.

## Success Criteria

- The patterns described in this document are applied correctly in the implementation.
- Edge cases and anti-patterns listed in this document are avoided.
