Relations in Prisma in NestJS

Introduction

Relations in Prisma help you connect different data tables easily. They let you link related information so you can get all connected data in one go.

When you want to link users with their posts in a blog app.

When you need to connect orders with customers in a shop system.

When you want to fetch comments related to a specific article.

When you want to organize data that belongs together, like products and categories.

Syntax

NestJS

model User {
  id    Int     @id @default(autoincrement())
  name  String
  posts Post[]  // One user has many posts
}

model Post {
  id       Int    @id @default(autoincrement())
  title    String
  content  String
  authorId Int
  author   User   @relation(fields: [authorId], references: [id])
}

Use arrays (e.g., Post[]) to show one-to-many relations.

The @relation attribute links fields between models.

Examples

Basic one-to-many relation: one user has many posts.

NestJS

model User {
  id    Int    @id @default(autoincrement())
  name  String
  posts Post[]
}

model Post {
  id       Int    @id @default(autoincrement())
  title    String
  authorId Int
  author   User   @relation(fields: [authorId], references: [id])
}

One category has many products linked by categoryId.

NestJS

model Category {
  id       Int      @id @default(autoincrement())
  name     String
  products Product[]
}

model Product {
  id          Int      @id @default(autoincrement())
  name        String
  categoryId  Int
  category    Category @relation(fields: [categoryId], references: [id])
}

One-to-one relation: a user has one profile.

NestJS

model Profile {
  id     Int  @id @default(autoincrement())
  bio    String
  userId Int  @unique
  user   User @relation(fields: [userId], references: [id])
}

model User {
  id      Int      @id @default(autoincrement())
  name    String
  profile Profile?
}

Sample Program

This code creates a user named Alice with two posts. It then prints the user name and all her posts.

NestJS

import { PrismaClient } from '@prisma/client';

const prisma = new PrismaClient();

async function main() {
  // Create a user
  const user = await prisma.user.create({
    data: {
      name: 'Alice',
      posts: {
        create: [
          { title: 'Hello World', content: 'My first post' },
          { title: 'Prisma is cool', content: 'Learning relations' }
        ]
      }
    },
    include: { posts: true }
  });

  console.log(`User: ${user.name}`);
  console.log('Posts:');
  user.posts.forEach(post => {
    console.log(`- ${post.title}: ${post.content}`);
  });
}

main()
  .catch(e => console.error(e))
  .finally(async () => {
    await prisma.$disconnect();
  });

Important Notes

Always define the foreign key field (like authorId) when using relations.

Use include in queries to fetch related data together.

Remember to disconnect Prisma client to avoid hanging connections.

Summary

Relations connect different models to organize related data.

Use arrays for one-to-many and single fields for one-to-one relations.

Prisma makes fetching connected data simple with include.

Practice

1. What type of relation in Prisma connects one record to many records in another model?

easy

A. One-to-many relation

B. One-to-one relation

C. Many-to-many relation

D. No relation

Solution

1. Step 1: Understand relation types in Prisma

   Prisma supports one-to-one, one-to-many, and many-to-many relations to connect models.

2. Step 2: Identify the relation connecting one record to many

   One-to-many means one record in a model relates to multiple records in another model.

3. Final Answer:

   One-to-many relation -> Option A

4. Quick Check:

   One record linked to many = One-to-many [OK]

Hint: One record to many records means one-to-many relation [OK]

Common Mistakes:

• Confusing one-to-one with one-to-many
• Thinking many-to-many is one-to-many
• Assuming no relation means one-to-many

2. Which of the following is the correct Prisma schema syntax for a one-to-one relation between User and Profile models?

easy

A. model User {\n id Int @id @default(autoincrement())\n profiles Profile[]\n}\n\nmodel Profile {\n id Int @id @default(autoincrement())\n user User\n}

B. model User {\n id Int @id @default(autoincrement())\n profile Profile? @relation(fields: [profileId], references: [id])\n profileId Int?\n}\n\nmodel Profile {\n id Int @id @default(autoincrement())\n user User?\n}

C. model User {\n id Int @id @default(autoincrement())\n profile Profile\n}\n\nmodel Profile {\n id Int @id @default(autoincrement())\n user User\n}

D. model User {\n id Int @id @default(autoincrement())\n profileId Int\n}\n\nmodel Profile {\n id Int @id @default(autoincrement())\n}

Solution

1. Step 1: Check one-to-one relation syntax

   One-to-one relations use a single optional foreign key field with @relation specifying fields and references.

2. Step 2: Verify the syntax

   The correct schema declares profile Profile? @relation(fields: [profileId], references: [id]) and profileId Int? in User, with user User? in Profile. This matches the pattern for optional one-to-one.

3. Final Answer:

   Schema with explicit optional FK and @relation -> Option B

4. Quick Check:

   One-to-one uses single foreign key with @relation [OK]

Hint: One-to-one needs single foreign key with @relation fields [OK]

Common Mistakes:

• Using array [] for one-to-one relation
• Missing @relation fields and references
• Not making foreign key optional

3. Given the Prisma models below, what will be the result of the query prisma.user.findUnique({ where: { id: 1 }, include: { posts: true } }) if user with id 1 has two posts?

model User {
  id    Int    @id @default(autoincrement())
  name  String
  posts Post[]
}

model Post {
  id      Int   @id @default(autoincrement())
  title   String
  userId  Int
  user    User  @relation(fields: [userId], references: [id])
}

medium

A. An error because 'include' is not valid here

B. An object with user data but 'posts' will be undefined

C. An object with user data and an array of two post objects under 'posts'

D. An array of posts only without user data

Solution

1. Step 1: Understand the Prisma query with include

   The query fetches a user by id and includes related posts as an array.

2. Step 2: Check relation and data

   User with id 1 has two posts, so 'posts' will be an array of two post objects inside the user object.

3. Final Answer:

   User object with posts array of two posts -> Option C

4. Quick Check:

   Include fetches related data as array [OK]

Hint: Include fetches related records as array or object [OK]

Common Mistakes:

• Expecting posts to be undefined without include
• Thinking include causes error
• Expecting only posts without user

4. Identify the error in this Prisma schema for a one-to-many relation between Author and Book models:

model Author {
  id    Int    @id @default(autoincrement())
  name  String
  books Book
}

model Book {
  id       Int    @id @default(autoincrement())
  title    String
  authorId Int
  author   Author @relation(fields: [authorId], references: [id])
}

medium

A. Author model is missing a primary key

B. Missing @relation on Author.books field

C. authorId should be optional in Book model

D. The 'books' field in Author should be an array (Book[]) not a single Book

Solution

1. Step 1: Check relation type from schema

   Author to Book is one-to-many, so Author.books must be an array of Book.

2. Step 2: Identify the mistake

   Author.books is declared as a single Book, which is incorrect for one-to-many.

3. Final Answer:

   'books' must be Book[] array -> Option D

4. Quick Check:

   One-to-many uses array field [OK]

Hint: One-to-many needs array [] on 'many' side field [OK]

Common Mistakes:

• Using single object instead of array for many side
• Forgetting @relation decorator
• Making foreign key optional incorrectly

5. You want to fetch all Order records with their related Customer data, but only include customers who have placed more than 5 orders. Which Prisma query approach correctly applies this filter using relations?

model Customer {
  id     Int     @id @default(autoincrement())
  name   String
  orders Order[]
}

model Order {
  id         Int      @id @default(autoincrement())
  total      Float
  customerId Int
  customer   Customer @relation(fields: [customerId], references: [id])
}

hard

A. Use findMany on Order with include: { customer: true } and a where on customer orders count > 5

B. Use findMany on Customer with where: { orders: { some: {} } } and include orders

C. Use findMany on Order with where: { customer: { orders: { count: { gt: 5 } } } } and include customer

D. Use findMany on Customer with include: { orders: true } and filter orders count > 5 in JavaScript

Solution

1. Step 1: Understand filtering by related model count

   Prisma allows filtering with nested conditions on related models using 'where' and relation filters.

2. Step 2: Identify correct query

   Using findMany on Order with include: { customer: true } and where: { customer: { orders: { _count: { gt: 5 } } } } fetches the desired orders with customers who have more than 5 orders.

3. Final Answer:

   findMany on Order with relational where filter and customer include -> Option A

4. Quick Check:

   Filter related model count in where clause [OK]

Hint: Filter related model count in where clause with include [OK]

Common Mistakes:

• Filtering in JavaScript instead of query
• Using incorrect nested where syntax
• Filtering on Customer without including orders count