Relations (OneToMany, ManyToOne, ManyToMany) in NestJS - Mini Project: Build & Apply

Relations (OneToMany, ManyToOne, ManyToMany) in NestJS

📖 Scenario: You are building a simple blog application backend using NestJS and TypeORM. You need to set up the database models to represent authors, posts, and categories with proper relationships.

🎯 Goal: Create three entities: Author, Post, and Category with these relations:- Each Author can have many Posts (OneToMany)- Each Post belongs to one Author (ManyToOne)- Each Post can have many Categorys and each Category can have many Posts (ManyToMany)

📋 What You'll Learn

Create an Author entity with id and name fields

Create a Post entity with id, title, and content fields

Create a Category entity with id and name fields

Set up OneToMany and ManyToOne relations between Author and Post

Set up ManyToMany relation between Post and Category with a join table

💡 Why This Matters

🌍 Real World

Blog platforms, content management systems, and many apps need to model relationships between users, posts, and tags or categories.

💼 Career

Understanding entity relations is essential for backend developers working with databases and ORMs like TypeORM in NestJS.

1

Create the Author entity

Create a NestJS entity class called Author with two properties: id as a primary generated column and name as a string column.

NestJS

# Create the Author entity class with id and name properties
# Your code here

Need a hint?

Use @Entity() decorator for the class, @PrimaryGeneratedColumn() for id, and @Column() for name.

2

Create the Post entity with author relation

Create a NestJS entity class called Post with id as a primary generated column, title and content as string columns. Add a ManyToOne relation to Author named author with the inverse side as posts.

NestJS

import { Entity, PrimaryGeneratedColumn, Column, ManyToOne } from 'typeorm';
import { Author } from './author.entity';

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

  @Column()
  title: string;

  @Column()
  content: string;

  // Add ManyToOne relation to Author here
  // Your code here
}

Need a hint?

Use @ManyToOne in Post and @OneToMany in Author to link them.

3

Create the Category entity

Create a NestJS entity class called Category with id as a primary generated column and name as a string column.

NestJS

# Create the Category entity class with id and name properties
# Your code here

Need a hint?

Use @Entity(), @PrimaryGeneratedColumn(), and @Column() decorators.

4

Add ManyToMany relation between Post and Category

In the Post entity, add a ManyToMany relation to Category named categories with a join table. In the Category entity, add the inverse ManyToMany relation to Post named posts.

NestJS

import { Entity, PrimaryGeneratedColumn, Column, ManyToMany, JoinTable } from 'typeorm';
import { Post } from './post.entity';

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

  @Column()
  name: string;

  // Add ManyToMany relation to Post here
  // Your code here
}

import { Entity, PrimaryGeneratedColumn, Column, ManyToOne, ManyToMany, JoinTable } from 'typeorm';
import { Author } from './author.entity';
import { Category } from './category.entity';

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

  @Column()
  title: string;

  @Column()
  content: string;

  @ManyToOne(() => Author, author => author.posts)
  author: Author;

  // Add ManyToMany relation to Category here
  // Your code here
}

Need a hint?

Use @ManyToMany on both sides and @JoinTable() on the owning side (Post).