Repository pattern in NestJS - Step-by-Step Execution

Concept Flow - Repository pattern

Start

↓

Service calls Repository

↓

Repository handles data logic

↓

Repository calls Database

↓

Database returns data

↓

Repository returns data to Service

↓

Service returns data to Controller

↓

Controller sends response

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End

The Repository pattern separates data access logic from business logic by having the Service call the Repository, which interacts with the Database and returns data back.

Execution Sample

NestJS

class UserRepository {
  findUser(id) {
    return db.find('users', id);
  }
}

class UserService {
  constructor(userRepository) {
    this.userRepository = userRepository;
  }

  getUser(id) { return this.userRepository.findUser(id); }
}

This code shows a Service calling a Repository method to get user data from the database.

Execution Table

Step	Action	Input	Process	Output
1	Controller receives request	userId=5	Passes to Service	Calls UserService.getUser(5)
2	Service calls Repository	id=5	Calls UserRepository.findUser(5)	Repository starts DB query
3	Repository queries DB	id=5	db.find('users', 5)	DB returns user data {id:5, name:'Anna'}
4	Repository returns data	{id:5, name:'Anna'}	Returns to Service	UserService.getUser returns user data
5	Service returns data	{id:5, name:'Anna'}	Returns to Controller	Controller sends response with user data
6	End	-	-	Response sent, process complete

💡 Process ends after Controller sends response to client

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
userId	undefined	5	5	5	5
userData	undefined	undefined	{id:5, name:'Anna'}	{id:5, name:'Anna'}	{id:5, name:'Anna'}

Key Moments - 2 Insights

Why does the Service call the Repository instead of directly querying the database?

The Service calls the Repository to keep business logic separate from data access. The Repository handles all database queries, making the code cleaner and easier to maintain, as shown in steps 2 and 3.

What happens if the Repository does not find the user in the database?

If the Repository finds no user, it returns null or undefined. The Service then handles this case before returning data to the Controller. This flow is implied between steps 3 and 4.

Visual Quiz - 3 Questions

Look at the execution table, what data does the Repository return to the Service at step 4?

A {id:5, name:'Anna'}

B userId=5

C null

D db.find('users', 5)

💡 Hint

Check the 'Output' column at step 4 in the execution table.

At which step does the database return the user data?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Process' and 'Output' columns to find when db.find returns data.

If the Service called the database directly, which step would be skipped?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

The Service calling the Repository is shown in step 2.

Concept Snapshot

Repository pattern separates data access from business logic.
Service calls Repository, Repository queries database.
Repository returns data to Service.
Keeps code clean and maintainable.
Common in NestJS for database operations.

Full Transcript

The Repository pattern in NestJS helps separate how data is fetched from how business rules are applied. The Controller receives a request and calls the Service. The Service then calls the Repository, which handles database queries. The database returns data to the Repository, which passes it back to the Service. Finally, the Controller sends the response. This separation keeps code organized and easier to maintain.