Relations in Prisma in NestJS - Mini Project: Build & Apply

Relations in Prisma with NestJS

📖 Scenario: You are building a simple blog backend using NestJS and Prisma. You want to model authors and their posts. Each author can have many posts, and each post belongs to one author.

🎯 Goal: Create Prisma models for Author and Post with a one-to-many relation. Then, write NestJS Prisma service code to fetch all posts with their authors.

📋 What You'll Learn

Create Prisma models Author and Post with a one-to-many relation

Add a config variable for Prisma client instance

Write a NestJS service method to fetch all posts including their author data

Complete the NestJS service class with Prisma client injection

💡 Why This Matters

🌍 Real World

Modeling and querying related data like users and their posts is common in web applications such as blogs, social media, and content management systems.

💼 Career

Understanding Prisma relations and integrating Prisma with NestJS is valuable for backend developers building scalable and maintainable APIs.

1

Define Prisma models for Author and Post

In your schema.prisma file, create two models: Author and Post. Define Author with fields id (Int, auto-increment, primary key) and name (String). Define Post with fields id (Int, auto-increment, primary key), title (String), and authorId (Int). Set up a one-to-many relation where Author has many posts and Post belongs to one author.

NestJS

// Define Prisma models Author and Post with one-to-many relation
// Your code here

Hint

Remember to use @relation attribute on the Post model to link to Author.

2

Create Prisma client instance in NestJS service

In your NestJS service file, import PrismaClient from @prisma/client and create a constant called prisma that is a new instance of PrismaClient.

NestJS

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

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

import { PrismaClient } from '@prisma/client';
// Create Prisma client instance
// Your code here

Hint

Use new PrismaClient() to create the client instance.

3

Write a method to fetch all posts with their authors

Inside your NestJS service class, write an async method called getAllPosts that returns all posts including their related author data. Use prisma.post.findMany with the include option to include author.

NestJS

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

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

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

const prisma = new PrismaClient();

export class PostService {
  // Write async method getAllPosts to fetch posts with authors
  // Your code here
}

Hint

Use include: { author: true } inside findMany to get author data.

4

Inject Prisma client into NestJS service class

Modify the PostService class to inject PrismaClient via the constructor. Add a private readonly property called prisma of type PrismaClient and assign it in the constructor parameter. Update the getAllPosts method to use this.prisma instead of the global prisma variable.

NestJS

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

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

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

const prisma = new PrismaClient();

export class PostService {
  // Add constructor to inject PrismaClient
  // Update getAllPosts to use this.prisma
  // Your code here
}

Hint

Use constructor injection to get PrismaClient instance and use this.prisma inside methods.

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