cjyyzx/.agents/skills/db-migrations/SKILL.md

name, description

name	description
db-migrations	Use when generating or regenerating Drizzle migration files, changing database schema tables or columns, resolving migration sequence conflicts after rebase, reviewing migration SQL for idempotent patterns, or renaming migration files.

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).