Building an E-commerce Backend: Auth, Cart, and Transactional Orders with Prisma

This is the second stage of my CodeAlpha Full Stack internship — two projects, built in a deliberate order so the patterns from the first carry forward. First was a project management tool (auth + real-time updates with Socket.io). This one is a store: products, cart, orders. Same stack — Express, Prisma, PostgreSQL, JWT — but the interesting part isn't the CRUD, it's the order-placement flow, which is the first genuinely transactional piece of logic in the whole internship. I'll walk through the schema decisions, the auth changes from project one, and then spend most of the time on the part that actually matters: making sure an order can never be created without correctly and atomically updating stock and clearing the cart. The schema model User { id String @id @default(cuid()) name String email String @unique password String role String @default("USER") createdAt DateTime @default(now()) orders Order[] cartItems CartItem[] } model Product { id String @id @default(cuid()) name String description String price Float image String? stock Int @default(0) category String createdAt DateTime @default(now()) cartItems CartItem[] orderItems OrderItem[] } model CartItem { id String @id @default(cuid()) quantity Int @default(1) user User @relation(fields: [userId], references: [id]) userId String product Product @relation(fields: [productId], references: [id]) productId String @@unique([userId, productId]) } model Order { id String @id @default(cuid()) status String @default("PENDING") total Float createdAt DateTime @default(now()) user User @relation(fields: [userId], references: [id]) userId String items OrderItem[] } model OrderItem { id String @id @default(cuid()) quantity Int price Float order Order @relation(fields: [orderId], references: [id]) orderId String product Product @relation(fields: [productId], references: [id]) productId String } Two decisions worth explaining, because they're easy to get wrong if you're building this for the first time. OrderItem.price is a