Entity definition in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS entity definition?

Consider this NestJS entity using TypeORM decorators. What will be the type of the 'id' property in the database?

NestJS

import { Entity, PrimaryGeneratedColumn, Column } from 'typeorm';

@Entity()
export class User {
  @PrimaryGeneratedColumn('uuid')
  id: string;

  @Column()
  name: string;
}

A A boolean column

B An auto-incrementing integer column

C A plain string column without auto-generation

D A UUID string column generated automatically

💡 Hint

Look at the argument passed to @PrimaryGeneratedColumn decorator.

The decorator @PrimaryGeneratedColumn('uuid') tells TypeORM to generate a UUID string automatically for the 'id' field.

📝 Syntax

intermediate

Which option correctly defines a nullable column in a NestJS entity?

You want to define a column 'description' that can be empty or null in the database. Which code snippet correctly achieves this?

NestJS

import { Entity, Column, PrimaryGeneratedColumn } from 'typeorm';

@Entity()
export class Product {
  @PrimaryGeneratedColumn()
  id: number;

  // Your code here
}

A

@Column({ nullable: true })
description: string;

B

@Column({ optional: true })
description: string;

C

@Column({ null: true })
description: string;

D

@Column({ allowNull: true })
description: string;

💡 Hint

Check the official TypeORM column options for nullable fields.

The correct option is 'nullable: true' to allow null values in the database column.

🔧 Debug

advanced

Why does this NestJS entity fail to save data?

Given this entity, why will saving a new instance cause an error?

NestJS

import { Entity, PrimaryGeneratedColumn, Column } from 'typeorm';

@Entity()
export class Order {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  total: number;

  @Column({ type: 'timestamp' })
  date: Date;
}

A The 'date' column is missing a transformer to convert Date to database format

B The entity is missing a constructor to initialize properties

C The 'date' column needs the 'type' option set explicitly to 'timestamp' or 'date'

D The 'date' property type should be 'string' instead of 'Date'

💡 Hint

TypeORM needs to know how to store Date objects in the database.

Without specifying the column type, TypeORM may not map the Date property correctly, causing errors on save.

🧠 Conceptual

advanced

What is the purpose of the @Entity decorator in NestJS with TypeORM?

Why do we use the @Entity decorator on a class in NestJS when working with TypeORM?

A It registers the class as a service injectable in NestJS

B It marks the class as a database table and enables ORM mapping

C It automatically creates REST API endpoints for the class

D It compiles the class into a standalone executable

💡 Hint

Think about how ORM knows which classes represent tables.

The @Entity decorator tells TypeORM that this class maps to a database table.

❓ state_output

expert

What is the value of 'user.isActive' after saving and retrieving this entity?

Consider this entity and the following code snippet. What will be the value of 'isActive' after saving and retrieving the user from the database?

NestJS

import { Entity, PrimaryGeneratedColumn, Column } from 'typeorm';

@Entity()
export class User {
  @PrimaryGeneratedColumn()
  id: number;

  @Column({ default: true })
  isActive: boolean;
}

// Usage:
const user = new User();
await repository.save(user);
const savedUser = await repository.findOneBy({ id: user.id });
const result = savedUser?.isActive;

A true

B false

C undefined

D null

💡 Hint

Check the default value set in the @Column decorator and what happens when no value is assigned before saving.

The 'isActive' column has a default value of true, so when a new User is saved without setting 'isActive', the database assigns true.