Prisma Client usage in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Prisma Client query in NestJS?

Consider a Prisma Client query fetching a user by ID. What will be the output if the user does not exist?

NestJS

const user = await prisma.user.findUnique({ where: { id: 999 } });
console.log(user);

A {} (empty object)

B undefined

C Throws an error

D null

💡 Hint

Think about what Prisma returns when no record matches the query.

Prisma Client returns null when findUnique finds no matching record.

❓ state_output

intermediate

What is the value of 'createdUser' after this Prisma create call?

Given the following Prisma Client code in a NestJS service, what does 'createdUser' contain?

NestJS

const createdUser = await prisma.user.create({
  data: { email: '[e-mail]', name: 'Test User' }
});
console.log(createdUser);

A { email: '[e-mail]', name: 'Test User' }

B { id: number, email: '[e-mail]', name: 'Test User', createdAt: Date }

C null

D Throws a runtime error

💡 Hint

Prisma returns the full created record including auto-generated fields.

The create method returns the full record including generated fields like id and createdAt.

📝 Syntax

advanced

Which option correctly updates a user's name using Prisma Client in NestJS?

Select the correct Prisma Client syntax to update the name of a user with id 5 to 'Alice'.

A await prisma.user.update({ where: { id: 5 }, data: { name: 'Alice' } });

B await prisma.user.update({ data: { name: 'Alice' }, where: { id: 5 } });

C await prisma.user.update({ id: 5, name: 'Alice' });

D await prisma.user.update({ where: { id: 5 }, name: 'Alice' });

💡 Hint

Check the order and keys required by Prisma's update method.

The update method requires where and data keys. The order of keys does not matter, but the structure must be correct.

🔧 Debug

advanced

What error does this Prisma Client code produce?

Analyze the following code snippet and identify the error it causes.

NestJS

const user = await prisma.user.findUnique({ where: { email: '[e-mail]' } });
console.log(user.name);

A No error, prints undefined

B ReferenceError: user is not defined

C TypeError: Cannot read property 'name' of null

D SyntaxError: Unexpected token '.'

💡 Hint

Consider what happens when findUnique returns null and you try to access a property.

If findUnique returns null, accessing user.name causes a TypeError.

🧠 Conceptual

expert

Which option best describes Prisma Client's transaction behavior in NestJS?

When using Prisma Client's $transaction method in NestJS, what happens if one query inside the transaction fails?

A All queries inside the transaction are rolled back; no changes are saved.

B Only the failed query is rolled back; others remain committed.

C The transaction ignores errors and commits all queries anyway.

D Prisma Client throws an error but commits successful queries.

💡 Hint

Think about the atomic nature of transactions.

Prisma Client's $transaction ensures atomicity: if any query fails, all changes are rolled back.

Practice

1. What is the main purpose of Prisma Client in a NestJS application?

easy

A. To interact with the database using simple JavaScript calls

B. To handle HTTP requests and routing

C. To manage application state in the frontend

D. To style components with CSS

Solution

1. Step 1: Understand Prisma Client's role

   Prisma Client is a tool that helps you talk to your database easily using JavaScript.

2. Step 2: Compare with other options

   Handling HTTP requests and routing, managing frontend state, and styling components relate to other parts of an app, not database interaction.

3. Final Answer:

   To interact with the database using simple JavaScript calls -> Option A

4. Quick Check:

   Prisma Client = database interaction [OK]

Hint: Prisma Client is for database queries, not UI or routing [OK]

Common Mistakes:

• Confusing Prisma Client with HTTP routing
• Thinking Prisma Client manages frontend state
• Assuming Prisma Client styles components

2. Which of the following is the correct way to import and instantiate Prisma Client in a NestJS service?

easy

A. import { PrismaClient } from '@prisma/client'; const prisma = new PrismaClient();

B. const prisma = require('prisma-client');

C. import Prisma from 'prisma'; const prisma = Prisma();

D. import { Prisma } from '@nestjs/prisma'; const prisma = new Prisma();

Solution

1. Step 1: Check the official import syntax

   The correct import is from '@prisma/client' and creating a new PrismaClient instance.

2. Step 2: Identify incorrect options

   Using require('prisma-client'), importing from 'prisma', or using '@nestjs/prisma' with incorrect instantiation do not match Prisma Client usage.

3. Final Answer:

   import { PrismaClient } from '@prisma/client'; const prisma = new PrismaClient(); -> Option A

4. Quick Check:

   Correct import and instantiation = import { PrismaClient } from '@prisma/client'; const prisma = new PrismaClient(); [OK]

Hint: Always import PrismaClient from '@prisma/client' and instantiate with new [OK]

Common Mistakes:

• Using require instead of import
• Wrong package names like 'prisma-client'
• Confusing NestJS modules with Prisma Client

3. Given this Prisma Client query in a NestJS service:

const user = await prisma.user.findUnique({ where: { id: 5 } });
console.log(user);

What will be logged if a user with id 5 exists in the database?

medium

A. An array of all users in the database

B. A syntax error

C. null

D. An object containing the user's data with id 5

Solution

1. Step 1: Understand findUnique behavior

   findUnique returns a single record matching the condition or null if none found.

2. Step 2: Given user with id 5 exists

   The query returns the user object with id 5, so console.log prints that object.

3. Final Answer:

   An object containing the user's data with id 5 -> Option D

4. Quick Check:

   findUnique with existing id = user object [OK]

Hint: findUnique returns one object or null, not array [OK]

Common Mistakes:

• Expecting an array instead of an object
• Assuming null even if user exists
• Thinking it causes syntax error

4. Identify the error in this NestJS Prisma Client usage:

import { PrismaClient } from '@prisma/client';
const prisma = new PrismaClient();

async function getUsers() {
  const users = prisma.user.findMany();
  return users;
}

medium

A. Function getUsers should not be async

B. Missing await before prisma.user.findMany()

C. findMany() does not exist on prisma.user

D. PrismaClient should not be instantiated directly

Solution

1. Step 1: Check async function usage

   prisma.user.findMany() returns a Promise, so it must be awaited inside async functions.

2. Step 2: Identify missing await

   The code misses 'await' before the call, so it returns a Promise instead of resolved data.

3. Final Answer:

   Missing await before prisma.user.findMany() -> Option B

4. Quick Check:

   Async DB calls need await [OK]

Hint: Await all Prisma async calls inside async functions [OK]

Common Mistakes:

• Forgetting await on async Prisma calls
• Thinking PrismaClient can't be instantiated
• Assuming findMany() is invalid

5. In a NestJS app, you want to ensure Prisma Client connects once on app start and disconnects on shutdown. Which approach is best?

hard

A. Call prisma.$connect() before every query and prisma.$disconnect() after every query

B. Create a new PrismaClient instance inside every controller method

C. Create PrismaClient instance in a service, call prisma.$connect() in onModuleInit(), and prisma.$disconnect() in onModuleDestroy()

D. Use PrismaClient without calling connect or disconnect methods

Solution

1. Step 1: Understand PrismaClient lifecycle

   PrismaClient should connect once when app starts and disconnect on shutdown to manage resources efficiently.

2. Step 2: Evaluate options

   Creating a single PrismaClient instance in a service and using $connect() in onModuleInit() and $disconnect() in onModuleDestroy() leverages NestJS lifecycle hooks properly. Connecting/disconnecting per query, creating new instances per method, or skipping explicit management leads to overhead or resource leaks.

3. Final Answer:

   Create PrismaClient instance in a service, call prisma.$connect() in onModuleInit(), and prisma.$disconnect() in onModuleDestroy() -> Option C

4. Quick Check:

   Use lifecycle hooks for Prisma connection [OK]

Hint: Connect once on start, disconnect on shutdown using lifecycle hooks [OK]

Common Mistakes:

• Creating PrismaClient multiple times
• Connecting/disconnecting on every query
• Ignoring connect/disconnect lifecycle