Repository pattern in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS repository method call?

Consider a NestJS repository method that fetches all users with the role 'admin'. What will be the output if the database contains 3 users with roles: 'admin', 'user', 'admin'?

NestJS

async findAdmins() {
  return this.userRepository.find({ where: { role: 'admin' } });
}

A An array with 2 user objects having role 'admin'

B An array with all 3 user objects regardless of role

C An empty array

D A single user object with role 'admin'

💡 Hint

Remember that the find method returns all matching records as an array.

The find method with a where clause returns all records matching the condition. Since 2 users have role 'admin', the output is an array of those 2 users.

📝 Syntax

intermediate

Which option correctly defines a custom repository method in NestJS?

You want to add a method 'findByEmail' to your UserRepository to find a user by email. Which code snippet correctly defines this method?

A

async findByEmail(email: string) {
  return this.findOneBy({ email });
}

B

async findByEmail(email: string) {
  return this.userRepository.findOneByEmail(email);
}

C

async findByEmail(email: string) {
  return this.userRepository.findOne({ email });
}

D

async findByEmail(email: string) {
  return this.findOne({ where: { email } });
}

💡 Hint

Check the correct TypeORM repository method for finding one record by condition.

The method findOneBy is the correct TypeORM repository method to find a single record by a condition object.

🔧 Debug

advanced

Why does this NestJS repository method throw a runtime error?

Given the method below, why does calling 'findUserById(5)' throw an error?

NestJS

async findUserById(id: number) {
  return this.userRepository.findOne({ where: { id } });
}

A The method findOne is deprecated and cannot be used

B The id parameter must be a string, not a number

C findOne expects an object with conditions, not a direct id argument

D The repository is not injected properly

💡 Hint

Check the TypeORM repository method signatures for findOne.

In TypeORM v0.3+, findOne requires an object with conditions like { where: { id } }, passing id directly causes a runtime error.

❓ state_output

advanced

What is the state of the database after this repository save operation?

If you call this method to save a user object with id 10 and name 'Alice', what happens if a user with id 10 already exists?

NestJS

async saveUser(user: User) {
  return this.userRepository.save(user);
}

// Called with { id: 10, name: 'Alice' }

A The existing user remains unchanged

B A new user with id 10 is created, causing a duplicate key error

C The save method throws a runtime error

D The existing user with id 10 is updated with name 'Alice'

💡 Hint

The save method updates if primary key exists, otherwise inserts.

TypeORM's save method updates the record if the primary key exists, so the user with id 10 is updated with the new name.

🧠 Conceptual

expert

Which statement best describes the role of the repository pattern in NestJS?

Choose the most accurate description of why the repository pattern is used in NestJS applications.

A It automatically generates REST API endpoints for database entities

B It abstracts data access logic, allowing business logic to remain independent of database details

C It replaces the need for service classes in NestJS

D It enforces strict typing on all database queries

💡 Hint

Think about separation of concerns between data access and business logic.

The repository pattern abstracts how data is fetched or saved, so the rest of the app does not depend on database specifics.