Relations (OneToMany, ManyToOne, ManyToMany) in NestJS - Performance & Optimization

Performance: Relations (OneToMany, ManyToOne, ManyToMany)

MEDIUM IMPACT

This affects database query performance and server response time, impacting how fast data loads and updates in the app.

Fetching related data with relations in NestJS using TypeORM

NestJS

const users = await userRepository.find({ relations: ['posts'] });

Fetches users and their posts in a single query using JOIN, reducing database calls.

📈 Performance GainSingle query regardless of number of users, greatly reducing response time.

Fetching related data with relations in NestJS using TypeORM

NestJS

const users = await userRepository.find();
for (const user of users) {
  user.posts = await postRepository.find({ where: { user: { id: user.id } } });
}

This triggers a separate database query for each user to get posts, causing many queries (N+1 problem).

📉 Performance CostTriggers N+1 queries, increasing server response time linearly with number of users.

Performance Comparison

Pattern	Database Queries	Query Count	Response Time Impact	Verdict
N+1 Query Pattern	Multiple separate queries	N+1 queries	High response time, slower LCP	[X] Bad
Eager Loading Relations	Single joined query	1 query	Low response time, faster LCP	[OK] Good
ManyToMany without indexes	Join queries on large tables	1 query	Potentially slow if unindexed	[!] OK
ManyToMany with indexed join table	Optimized join query	1 query	Fast response, scalable	[OK] Good

Rendering Pipeline

Relations affect server-side data fetching before rendering. Efficient queries reduce server response time, improving initial content paint.

→Data Fetching

→Server Response

→Initial Render

⚠️ BottleneckDatabase query time due to inefficient relation loading

Core Web Vital Affected

LCP

This affects database query performance and server response time, impacting how fast data loads and updates in the app.

Optimization Tips

1Avoid N+1 query problem by using eager loading relations.

2Use explicit join tables with indexes for ManyToMany relations.

3Fetch related data in as few queries as possible to reduce server response time.

Performance Quiz - 3 Questions

What is the main performance problem with fetching related data in a loop without using relations?

A It reduces the number of queries, speeding up response

B It causes many database queries, slowing down response time

C It caches data automatically, improving performance

D It compresses data to reduce network size

DevTools: Network and Performance panels

How to check: Open DevTools, go to Network tab, filter XHR requests, observe API calls for data fetching. Use Performance tab to record page load and check server response times.

What to look for: Look for multiple API calls fetching related data separately (bad). Single API call with joined data (good). Faster server response times improve LCP.