Database Migrations Guide Skill

Database Migrations Guide

Step 1: Generate Migrations

bun run db:generate

This generates:

packages/database/migrations/0046_meaningless_file_name.sql

And updates:

packages/database/migrations/meta/_journal.json
packages/database/src/core/migrations.json
docs/development/database-schema.dbml

Custom Migrations (e.g. CREATE EXTENSION)

For migrations that don't involve Drizzle schema changes (e.g. enabling PostgreSQL extensions), use the --custom flag:

bunx drizzle-kit generate --custom --name=enable_pg_search

This generates an empty SQL file and properly updates _journal.json and snapshot. Then edit the generated SQL file to add your custom SQL:

-- Custom SQL migration file, put your code below! --
CREATE EXTENSION IF NOT EXISTS pg_search;

Do NOT manually create migration files or edit _journal.json — always use drizzle-kit generate to ensure correct journal entries and snapshots.

Step 2: Optimize Migration SQL Filename

Rename auto-generated filename to be meaningful:

0046_meaningless_file_name.sql → 0046_user_add_avatar_column.sql

Step 3: Use Idempotent Clauses (Defensive Programming)

Always use defensive clauses to make migrations idempotent (safe to re-run):

CREATE TABLE

-- ✅ Good
CREATE TABLE IF NOT EXISTS "agent_eval_runs" (
  "id" text PRIMARY KEY NOT NULL,
  "name" text,
  "created_at" timestamp with time zone DEFAULT now() NOT NULL
);

-- ❌ Bad
CREATE TABLE "agent_eval_runs" (...);

ALTER TABLE - Columns

-- ✅ Good
ALTER TABLE "users" ADD COLUMN IF NOT EXISTS "avatar" text;
ALTER TABLE "posts" DROP COLUMN IF EXISTS "deprecated_field";

-- ❌ Bad
ALTER TABLE "users" ADD COLUMN "avatar" text;

ALTER TABLE - Foreign Key Constraints

PostgreSQL has no ADD CONSTRAINT IF NOT EXISTS. Use DROP IF EXISTS + ADD:

-- ✅ Good: Drop first, then add (idempotent)
ALTER TABLE "agent_eval_datasets" DROP CONSTRAINT IF EXISTS "agent_eval_datasets_user_id_users_id_fk";
ALTER TABLE "agent_eval_datasets" ADD CONSTRAINT "agent_eval_datasets_user_id_users_id_fk"
  FOREIGN KEY ("user_id") REFERENCES "public"."users"("id") ON DELETE cascade ON UPDATE no action;

-- ❌ Bad: Will fail if constraint already exists
ALTER TABLE "agent_eval_datasets" ADD CONSTRAINT "agent_eval_datasets_user_id_users_id_fk"
  FOREIGN KEY ("user_id") REFERENCES "public"."users"("id") ON DELETE cascade ON UPDATE no action;

DROP TABLE / INDEX

-- ✅ Good
DROP TABLE IF EXISTS "old_table";
CREATE INDEX IF NOT EXISTS "users_email_idx" ON "users" ("email");
CREATE UNIQUE INDEX IF NOT EXISTS "users_email_unique" ON "users" USING btree ("email");

-- ❌ Bad
DROP TABLE "old_table";
CREATE INDEX "users_email_idx" ON "users" ("email");

Step 4: Update Journal Tag

After renaming the migration SQL file in Step 2, update the tag field in packages/database/migrations/meta/_journal.json to match the new filename (without .sql extension).

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