database-patterns

Database Patterns — TypeScript Cheat Sheet

Repository Pattern with Generics

Abstract data access behind a typed interface. Keeps business logic ORM-agnostic.

interface Repository<T, CreateDto, UpdateDto> {
  findById(id: string): Promise<T | null>;
  findMany(filter?: Partial<T>): Promise<T[]>;
  create(data: CreateDto): Promise<T>;
  update(id: string, data: UpdateDto): Promise<T>;
  delete(id: string): Promise<void>;
}

Prisma

Schema

model User {
  id        String   @id @default(uuid())
  email     String   @unique
  name      String
  orders    Order[]
  createdAt DateTime @default(now())
}

Typed Repository

import { PrismaClient, User, Prisma } from "@prisma/client";

class PrismaUserRepo implements Repository<User, Prisma.UserCreateInput, Prisma.UserUpdateInput> {
  constructor(private prisma: PrismaClient) {}

  findById(id: string) { return this.prisma.user.findUnique({ where: { id } }); }
  findMany() { return this.prisma.user.findMany(); }
  create(data: Prisma.UserCreateInput) { return this.prisma.user.create({ data }); }
  update(id: string, data: Prisma.UserUpdateInput) {
    return this.prisma.user.update({ where: { id }, data });
  }
  async delete(id: string) { await this.prisma.user.delete({ where: { id } }); }
}

N+1 Prevention (Prisma)

// WRONG: N+1 — fetches orders per user in a loop
const users = await prisma.user.findMany();
for (const u of users) { u.orders = await prisma.order.findMany({ where: { userId: u.id } }); }

// CORRECT: Eager load with include
const users = await prisma.user.findMany({ include: { orders: true } });

Drizzle

Schema & Select

import { pgTable, uuid, text, timestamp } from "drizzle-orm/pg-core";

export const users = pgTable("users", {
  id: uuid("id").primaryKey().defaultRandom(),
  email: text("email").notNull().unique(),
  name: text("name").notNull(),
  createdAt: timestamp("created_at").defaultNow(),
});

// Typed query
const result = await db.select().from(users).where(eq(users.email, "[email protected]"));
// result is typeof { id: string; email: string; name: string; createdAt: Date }[]

Joins (N+1 Prevention)

const usersWithOrders = await db
  .select()
  .from(users)
  .leftJoin(orders, eq(users.id, orders.userId));

Knex (Typed Query Builder)

interface UserRow { id: string; email: string; name: string; created_at: Date; }

const users = await knex<UserRow>("users")
  .where("email", "like", "%@example.com")
  .orderBy("created_at", "desc")
  .limit(20);

Migration Best Practices

1. One concern per migration — do not mix schema changes with data backfills.
2. Always provide a down — reversible migrations save production incidents.
3. Non-destructive first — add new column, backfill, switch reads, then drop old column.
4. Use transactions for DDL when the database supports it (Postgres does, MySQL does not for DDL).
5. Name migrations descriptively: 20240315_add_users_email_index.ts.

Connection Pooling

// Prisma — set pool in connection string
// postgresql://user:pass@host:5432/db?connection_limit=20&pool_timeout=10

// Knex
const knex = require("knex")({
  client: "pg",
  connection: { /* ... */ },
  pool: { min: 2, max: 20, idleTimeoutMillis: 30000 },
});

Rules of thumb:

• Pool size = (core_count * 2) + spindle_count (for traditional setups)
• Serverless: use external poolers (PgBouncer, Prisma Accelerate, Neon pooler)
• Always set pool_timeout / idleTimeoutMillis to avoid leaked connections

Transactions

// Prisma interactive transaction
await prisma.$transaction(async (tx) => {
  const user = await tx.user.create({ data: userData });
  await tx.order.create({ data: { ...orderData, userId: user.id } });
});

// Knex transaction
await knex.transaction(async (trx) => {
  const [user] = await trx("users").insert(userData).returning("*");
  await trx("orders").insert({ ...orderData, user_id: user.id });
});

Quick Rules

• Always parameterize queries — never interpolate user input into SQL
• Index foreign keys and columns used in WHERE/ORDER BY
• Use EXPLAIN ANALYZE to verify query plans before shipping
• Prefer select over select * — fetch only needed columns
• Soft-delete (deleted_at timestamp) for audit-critical tables