Entity definition in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an Entity in NestJS when using TypeORM?

An Entity is a class that maps to a database table. It defines the structure of the data and how it is stored in the database.

beginner

How do you mark a class as an Entity in NestJS with TypeORM?

You use the @Entity() decorator above the class to tell TypeORM this class represents a database table.

beginner

What decorator is used to define a primary key column in a NestJS Entity?

The @PrimaryGeneratedColumn() decorator is used to define a primary key that auto-increments.

beginner

How do you define a regular column in a NestJS Entity?

Use the @Column() decorator above a class property to define a regular database column.

beginner

Why is it important to define Entities in NestJS applications?

Entities help organize and manage how data is saved and retrieved from the database, making code easier to understand and maintain.

Which decorator marks a class as a database table in NestJS with TypeORM?

A @Entity()

B @Column()

C @Injectable()

D @Controller()

The @Entity() decorator tells TypeORM that the class is a database table.

What decorator is used to create an auto-incrementing primary key column?

A @PrimaryColumn()

B @Column()

C @PrimaryGeneratedColumn()

D @Generated()

@PrimaryGeneratedColumn() creates a primary key that auto-increments.

How do you define a normal column in an Entity?

A @Entity()

B @Column()

C @PrimaryGeneratedColumn()

D @Injectable()

The @Column() decorator defines a regular column in the database table.

What does an Entity class represent in NestJS with TypeORM?

A A service for business logic

B A middleware function

C A controller for routes

D A database table structure

An Entity class represents the structure of a database table.

Why should you use Entities in your NestJS app?

A To organize database data structure

B To handle HTTP requests

C To style the UI

D To manage user sessions

Entities organize how data is stored and retrieved from the database.

Explain how to define an Entity in NestJS using TypeORM and what decorators are essential.

Think about how a class becomes a table and how columns are defined.

Describe why Entities are important in a NestJS application that uses a database.

Consider how data is saved and retrieved in an app.

Practice

1. What is the purpose of the @Entity() decorator in a NestJS application using TypeORM?

easy

A. It marks a class as a database table representation.

B. It defines a method to fetch data from the database.

C. It creates a new database connection.

D. It styles the output of a component.

Solution

1. Step 1: Understand the role of @Entity()

   The @Entity() decorator tells NestJS and TypeORM that the class represents a database table.

2. Step 2: Differentiate from other decorators

   Other decorators like @Column() define table columns, but @Entity() marks the whole class as a table.

3. Final Answer:

   It marks a class as a database table representation. -> Option A

4. Quick Check:

   @Entity() = Table marker [OK]

Hint: Remember: @Entity() means 'this class is a table' [OK]

Common Mistakes:

• Confusing @Entity() with database connection setup
• Thinking @Entity() defines methods or styles
• Mixing up @Entity() with @Column()

2. Which of the following is the correct way to define a string column named title in a NestJS entity using TypeORM?

easy

A. @Column({ type: 'number' }) title: string;

B. @Column('boolean') title: string;

C. @Column() title: string;

D. @Column({ length: 255 }) title: number;

Solution

1. Step 1: Identify the correct @Column() usage for a string

   By default, @Column() without options assumes a string type in TypeORM.

2. Step 2: Check type consistency

   The property type should be string to match the column type.

3. Final Answer:

   @Column() title: string; -> Option C

4. Quick Check:

   Default @Column() = string column [OK]

Hint: Use @Column() alone for simple string columns [OK]

Common Mistakes:

• Setting wrong type in @Column() options
• Mismatching property type and column type
• Using number type for string properties

3. Given the following entity code, what will be the type of the age column in the database?

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

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

  @Column()
  name: string;

  @Column({ type: 'int' })
  age: number;
}

medium

A. INTEGER (number)

B. VARCHAR (string)

C. BOOLEAN

D. DATE

Solution

1. Step 1: Check the @Column() decorator on age

   The decorator specifies { type: 'int' }, which means an integer column in the database.

2. Step 2: Match the property type

   The property age is a number, matching the integer database type.

3. Final Answer:

   INTEGER (number) -> Option A

4. Quick Check:

   { type: 'int' } means integer column [OK]

Hint: Look for { type: 'int' } to identify integer columns [OK]

Common Mistakes:

• Assuming default string type without checking options
• Confusing int with boolean or date
• Ignoring the explicit type option in @Column()

4. Identify the error in the following entity definition:

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

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

  @Column({ type: 'varchar', length: 100 })
  name: number;
}

medium

A. Missing @Entity() decorator

B. Property type number does not match column type varchar

C. Primary key column missing

D. Incorrect import statement

Solution

1. Step 1: Check property and column type consistency

   The name property is typed as number but the column type is varchar, which expects a string.

2. Step 2: Confirm other parts are correct

   @Entity() is present, primary key is defined, and imports are correct.

3. Final Answer:

   Property type number does not match column type varchar -> Option B

4. Quick Check:

   Property type must match column type [OK]

Hint: Match property type with @Column() type options [OK]

Common Mistakes:

• Using wrong property type for string columns
• Forgetting to add @Entity()
• Missing primary key decorator

5. You want to define an entity Order with an auto-generated primary key id, a string customerName limited to 150 characters, and an optional numeric discount that can be null. Which is the correct entity definition?

hard

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

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

@Column({ length: 150 })
customerName: string;

@Column({ type: 'int', nullable: false })
discount: number;
}

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

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

@Column({ length: 150 })
customerName: string;

@Column({ type: 'float' })
discount: number;
}

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

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

@Column({ type: 'varchar', length: 150 })
customerName: number;

@Column({ type: 'float', nullable: true })
discount: number;
}

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

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

@Column({ length: 150 })
customerName: string;

@Column({ type: 'float', nullable: true })
discount?: number;
}

Solution

1. Step 1: Verify primary key definition

   @PrimaryGeneratedColumn() on id: number creates an auto-incrementing integer primary key. A string type would be incorrect without additional configuration like UUID.

2. Step 2: Check customerName

   @Column({ length: 150 }) with string type creates a VARCHAR(150) column. A number type mismatches.

3. Step 3: Confirm discount

   @Column({ type: 'float', nullable: true }) with optional discount?: number allows null values. Missing nullable: true, wrong type like 'int', or non-optional fails.

4. Final Answer:

   The entity with number id, string customerName({length:150}), optional nullable float discount -> Option D

5. Quick Check:

   Match types, nullable, and optional properties carefully [OK]

Hint: Use nullable: true and optional property for nullable columns [OK]

Common Mistakes:

• Mismatching property types and column types
• Forgetting to mark nullable columns as optional
• Using wrong primary key type