Use DTOs at Every Architectural Boundary

Use DTOs at Every Architectural Boundary

Impact: CRITICAL

Database types should not leak to the frontend. This has become a popular shortcut in our tech stack, but it's a code smell that creates multiple problems.

Problems with leaking database types:

• Technology coupling (Prisma types end up in React components)
• Security risks (accidental leakage of sensitive fields)
• Fragile contracts between server and client
• Inability to evolve the database schema independently

Incorrect (database types leaking):

// API route returning Prisma types directly
import type { User } from "@prisma/client";

export async function GET(): Promise<User> {
  const user = await prisma.user.findFirst();
  return user; // Leaks all database fields including sensitive ones
}

// Frontend using Prisma types
import type { User } from "@prisma/client";

function UserProfile({ user }: { user: User }) {
  // Component now coupled to database schema
}

Correct (explicit DTOs):

// Define explicit DTOs
interface UserDTO {
  id: number;
  name: string;
  email: string;
  // Only fields needed by the client
}

// API route transforms to DTO
export async function GET(): Promise<UserDTO> {
  const user = await userRepository.findById(id);
  return UserResponseSchema.parse(user); // Validate with Zod
}

// Frontend uses DTO
function UserProfile({ user }: { user: UserDTO }) {
  // Component decoupled from database
}

The standard:

1. Data layer → Application layer → API: Transform database models into application-layer DTOs, then transform application DTOs into API-specific DTOs
2. API → Application layer → Data layer: Transform API DTOs through application layer and into data-specific DTOs
3. All DTO conversions through Zod to ensure all data is validated before sending to user

DTO Location and Naming

Location: All DTOs go in packages/lib/dto/

Naming conventions:

• Base entity: {Entity}Dto (e.g., BookingDto)
• With relations: {Entity}With{Relations}Dto (e.g., BookingWithAttendeesDto)
• For specific projections: {Entity}For{Purpose}Dto (e.g., BookingForConfirmationDto)
• Avoid: {Entity}Dto2, {Entity}DtoForHandler, or other use-case-specific names

Enum/union pattern - use string literal unions to stay ORM-agnostic:

// Good - ORM-agnostic string literal union
export type BookingStatusDto = "CANCELLED" | "ACCEPTED" | "REJECTED" | "PENDING";

// Bad - importing Prisma enum
import { BookingStatus } from "@calcom/prisma/client";

Type safety - never use as any in DTO mapping functions. If types don't align, fix the mapping explicitly.

Prisma Boundaries

• Allowed: packages/prisma, repository implementations (packages/features/**/repositories/*Repository.ts), and low-level data access infrastructure.
• Not allowed: packages/features/** business logic (non-repository), packages/trpc/** handlers, apps/web/**, apps/api/v2/** services/controllers, and workflow/webhook/service layers.

Yes, this requires more code. Yes, it's worth it. Explicit boundaries prevent the architectural erosion that creates long-term maintenance nightmares.

Reference: Cal.com Engineering Blog