Schema definition in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS schema validation?

Given this schema and input, what will be the validation result?

NestJS

import { Prop, Schema, SchemaFactory } from '@nestjs/mongoose';
import { Document } from 'mongoose';

@Schema()
export class User extends Document {
  @Prop({ required: true })
  name: string;

  @Prop({ default: 18 })
  age: number;
}

const UserSchema = SchemaFactory.createForClass(User);

// Input data:
const input = { name: 'Alice' };

A Validation fails because age is missing

B Validation passes, age is set to 18 by default

C Validation passes, age is undefined

D Validation fails because name is missing

💡 Hint

Check the @Prop decorator options for default and required

The @Prop({ default: 18 }) sets age to 18 if missing. Name is required, and provided, so validation passes.

📝 Syntax

intermediate

Which option correctly defines a nested schema in NestJS?

You want to define a nested Address schema inside a User schema. Which code snippet is correct?

A

class Address {
  @Prop() street: string;
  @Prop() city: string;
}

@Schema()
class User {
  @Prop({ type: () => Address }) address: Address;
}

B

@Schema()
class Address {
  @Prop() street: string;
  @Prop() city: string;
}

@Schema()
class User {
  @Prop({ type: Address }) address: Address;
}

C

@Schema()
class Address {
  @Prop() street: string;
  @Prop() city: string;
}

@Schema()
class User {
  @Prop({ type: () => Address }) address: Address;
}

D

class Address {
  @Prop() street: string;
  @Prop() city: string;
}

@Schema()
class User {
  @Prop({ type: Address }) address: Address;
}

💡 Hint

Nested schemas require both classes to be decorated with @Schema and use a function for type.

Both Address and User must be decorated with @Schema(). The @Prop for nested schema uses type: () => Address to avoid circular reference issues.

🔧 Debug

advanced

Why does this NestJS schema throw a runtime error?

This schema code throws an error when creating a Product document. What is the cause?

NestJS

import { Prop, Schema, SchemaFactory } from '@nestjs/mongoose';
import { Document } from 'mongoose';

@Schema()
export class Product extends Document {
  @Prop()
  name: string;

  @Prop({ type: Number, required: true })
  price: number;
}

export const ProductSchema = SchemaFactory.createForClass(Product);

// Error: Missing required property 'price' when creating a Product document

A The 'price' property is missing a default value and is required, causing error if not provided

B The @Prop decorator must not include 'type' when using TypeScript types

C The class should not extend Document when using SchemaFactory

D The 'name' property is missing 'required: true' causing schema validation failure

💡 Hint

Check which properties are required and if defaults exist

The 'price' property is marked as required but has no default. If a new Product is created without 'price', Mongoose throws an error.

❓ state_output

advanced

What is the value of 'createdAt' after saving this schema?

Given this schema with timestamps enabled, what will be the value of 'createdAt' after saving a new document?

NestJS

import { Prop, Schema, SchemaFactory } from '@nestjs/mongoose';
import { Document } from 'mongoose';

@Schema({ timestamps: true })
export class Article extends Document {
  @Prop() title: string;
}

const ArticleSchema = SchemaFactory.createForClass(Article);

// After saving a new Article with title 'Hello', what is article.createdAt?

A Null because timestamps are disabled by default

B Undefined because 'createdAt' is not defined in the class

C A string with the ISO date format

D A Date object representing the creation time

💡 Hint

Timestamps option automatically adds createdAt and updatedAt fields as Date objects

With timestamps: true, Mongoose adds createdAt and updatedAt as Date objects automatically.

🧠 Conceptual

expert

Which option best explains the purpose of SchemaFactory in NestJS?

Why does NestJS use SchemaFactory.createForClass instead of directly using Mongoose schemas?

A It generates a Mongoose schema from a decorated TypeScript class, enabling decorators to define schema properties

B It replaces the need for Mongoose entirely by providing a custom schema system

C It compiles TypeScript classes into JavaScript functions for faster runtime performance

D It automatically connects the schema to the database without manual model creation

💡 Hint

Think about how decorators and classes relate to Mongoose schemas

SchemaFactory.createForClass converts a TypeScript class decorated with @Schema and @Prop into a Mongoose schema, bridging NestJS decorators with Mongoose.