Schema definition in NestJS

Introduction

A schema defines the shape and rules for data in your app. It helps NestJS know what data looks like and how to store it.

When you want to create a model for data stored in a database.

When you need to validate data structure before saving it.

When you want to organize data fields and types clearly.

When you want to connect your app with MongoDB using Mongoose.

When you want to reuse data structure rules across your app.

Syntax

NestJS

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

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

  @Prop()
  description?: string;

  @Prop({ default: 0 })
  quantity: number;
}

export const ItemSchema = SchemaFactory.createForClass(Item);

Use @Schema() to mark a class as a schema.

Use @Prop() to define each field with options like required or default.

Examples

This schema defines a User with required username and email, and an optional age.

NestJS

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

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

  @Prop({ required: true })
  email: string;

  @Prop()
  age?: number;
}

export const UserSchema = SchemaFactory.createForClass(User);

This schema adds automatic timestamps for created and updated times.

NestJS

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

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

  @Prop()
  content?: string;
}

export const PostSchema = SchemaFactory.createForClass(Post);

Sample Program

This example shows a Product schema with a required name, a stock number defaulting to 0, and an optional description.

NestJS

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

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

  @Prop({ default: 0 })
  stock: number;

  @Prop()
  description?: string;
}

export const ProductSchema = SchemaFactory.createForClass(Product);

Important Notes

Schema classes extend Document to get MongoDB document features.

Use SchemaFactory.createForClass() to generate the schema from the class.

You can add options like timestamps to track creation and update times automatically.

Summary

Schema definition sets the shape and rules for data in NestJS apps.

Use @Schema() and @Prop() decorators to build schemas.

Schemas help organize, validate, and store data consistently.

Practice

1. What is the main purpose of using @Schema() in NestJS?

easy

A. To define the structure and rules of a data model

B. To create a new controller

C. To handle HTTP requests

D. To style components

Solution

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

   @Schema() is used to define how data should be structured and validated in NestJS models.

2. Step 2: Compare with other options

   Creating controllers, handling HTTP requests, and styling components are unrelated to schema definition.

3. Final Answer:

   To define the structure and rules of a data model -> Option A

4. Quick Check:

   Schema decorator defines data structure [OK]

Hint: Remember: @Schema() sets data shape, not controllers or styles [OK]

Common Mistakes:

• Confusing @Schema() with controller decorators
• Thinking @Schema() handles HTTP logic
• Mixing schema with UI styling

2. Which of the following is the correct way to define a string property named title in a NestJS schema?

easy

A. @Prop() title: number;

B. @Prop() title: string;

C. @Prop() title = 123;

D. @Prop() title: boolean;

Solution

1. Step 1: Identify the correct type for a string property

   The property title should be of type string to hold text data.

2. Step 2: Check the syntax of @Prop()

   The correct syntax is @Prop() title: string; which declares a string property.

3. Final Answer:

   @Prop() title: string; -> Option B

4. Quick Check:

   String property uses type string [OK]

Hint: Use @Prop() with correct TypeScript type for properties [OK]

Common Mistakes:

• Using wrong types like number or boolean for string data
• Assigning values instead of declaring types
• Missing @Prop() decorator

3. Given this schema snippet:

@Schema()
export class Book {
  @Prop()
  author: string;

  @Prop({ required: true })
  pages: number;
}

What happens if you try to save a Book without pages?

medium

A. The save will throw a syntax error

B. The save will succeed and pages will be undefined

C. The save will succeed and pages will default to 0

D. The save will fail because pages is required

Solution

1. Step 1: Understand the required: true option

   The pages property is marked as required, so it must be provided when saving.

2. Step 2: Predict behavior when missing required property

   Omitting pages causes validation to fail and the save operation to be rejected.

3. Final Answer:

   The save will fail because pages is required -> Option D

4. Quick Check:

   Required props must be present [OK]

Hint: Required props block save if missing [OK]

Common Mistakes:

• Assuming missing required props default silently
• Confusing validation errors with syntax errors
• Ignoring the required option in @Prop()

4. Identify the error in this schema definition:

@Schema()
export class User {
  @Prop({ type: String })
  age: number;
}

medium

A. No error, this is correct

B. Missing @Schema() decorator

C. The type in @Prop() does not match the property type

D. Property name cannot be 'age'

Solution

1. Step 1: Compare @Prop() type and property type

   The decorator says type: String but the property is declared as number.

2. Step 2: Understand type mismatch consequences

   This mismatch can cause runtime errors or incorrect schema behavior.

3. Final Answer:

   The type in @Prop() does not match the property type -> Option C

4. Quick Check:

   Decorator type must match property type [OK]

Hint: Match @Prop() type with property type exactly [OK]

Common Mistakes:

• Using wrong type in @Prop() options
• Ignoring type mismatch warnings
• Assuming property name causes errors

5. How would you define a schema for a Product with a required name (string), optional price (number), and a default inStock (boolean) set to true?

hard

A. @Schema()
export class Product {
@Prop({ required: true })
name: string;

@Prop()
price?: number;

@Prop({ default: true })
inStock: boolean;
}

B. @Schema()
export class Product {
@Prop()
name: string;

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

@Prop({ default: false })
inStock: boolean;
}

C. @Schema()
export class Product {
@Prop({ required: true })
name: number;

@Prop()
price: string;

@Prop({ default: true })
inStock: boolean;
}

D. @Schema()
export class Product {
@Prop({ required: true })
name: string;

@Prop({ default: 0 })
price: number;

@Prop()
inStock: boolean;
}

Solution

1. Step 1: Define required name as string

   Use @Prop({ required: true }) name: string; to make name mandatory.

2. Step 2: Define optional price as number

   Use @Prop() price?: number; to allow price to be optional.

3. Step 3: Define inStock with default true

   Use @Prop({ default: true }) inStock: boolean; to set default value.

4. Final Answer:

   Schema with required name (string), optional price (number), and inStock default true -> Option A

5. Quick Check:

   Required, optional, default props set properly [OK]

Hint: Use required, optional ?, and default in @Prop() [OK]

Common Mistakes:

• Mixing types for properties
• Forgetting to mark required props
• Setting wrong default values