Entity definition in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a basic entity class with a decorator.

NestJS

import { [1] } from 'typeorm';

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

  @Column()
  name: string;
}

Drag options to blanks, or click blank then click option'

AController

BEntity

CModule

DInjectable

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Controller or @Module instead of @Entity

Forgetting to import the decorator from 'typeorm'

The @Entity() decorator marks the class as a database entity in TypeORM, which NestJS uses for database models.

2fill in blank

medium

Complete the code to define a primary key column in the entity.

NestJS

import { Entity, [1], Column } from 'typeorm';

@Entity()
export class Product {
  @[1]()
  id: number;

  @Column()
  title: string;
}

Drag options to blanks, or click blank then click option'

APrimaryGeneratedColumn

BPrimaryColumn

CGenerated

DCreateDateColumn

Attempts:

3 left

💡 Hint

Common Mistakes

Using @PrimaryColumn without generation

Confusing with @CreateDateColumn which is for timestamps

@PrimaryGeneratedColumn() creates a primary key column with auto-generated values, commonly used for IDs.

3fill in blank

hard

Fix the error in the entity property decorator to correctly define a string column.

NestJS

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

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

  @Column({ type: '[1]' })
  name: string;
}

Drag options to blanks, or click blank then click option'

Avarchar

Binteger

Cboolean

Ddate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'integer' or 'boolean' for string fields

Leaving type undefined causing default assumptions

The varchar type is used for string columns in SQL databases, which matches the string type in TypeScript.

4fill in blank

hard

Fill both blanks to define a nullable column with a default value.

NestJS

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

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

  @Column({ [1]: true, [2]: 'draft' })
  status: string;
}

Drag options to blanks, or click blank then click option'

Anullable

Bdefault

Cunique

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'unique' or 'length' instead of 'nullable' or 'default'

Forgetting to set nullable true for optional fields

nullable: true allows the column to have null values. default: 'draft' sets the default value when none is provided.

5fill in blank

hard

Fill all three blanks to define a one-to-many relationship between entities.

NestJS

import { Entity, PrimaryGeneratedColumn, Column, [1], [2], [3] } from 'typeorm';

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

  @Column()
  name: string;

  @[1](() => Post, post => post.author)
  posts: Post[];
}

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

  @Column()
  title: string;

  @[2](() => Author, author => author.posts)
  @[3]()
  author: Author;
}

Drag options to blanks, or click blank then click option'

AOneToMany

BManyToOne

CJoinColumn

DManyToMany

Attempts:

3 left

💡 Hint

Common Mistakes

Using ManyToMany instead of OneToMany/ManyToOne

Forgetting to import JoinColumn or use it on the owning side

OneToMany and ManyToOne decorators define the two sides of a one-to-many relationship. JoinColumn is used on the many-to-one side to specify the owning side of the relation.

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