A significant advantage of SQL databases is handling relationships between various tables. Since, in web applications, entities often relate to each other, designing relationships is a big part of working with SQL databases. In this article, we continue learning MikroORM and use it to form relationships. You can find the code from this article in this repository. One-To-One With a one-to-one relationship, a row from the first table has just one matching row from the second table and the other way around. An elementary example of that is creating an address entity. address.entity.ts import { Entity, Property, PrimaryKey } from '@mikro-orm/core'; @Entity() class Address { @PrimaryKey() id: number; @Property() street: string; @Property() city: string; @Property() country: string; } export default Address;Once we do that, we can use the @OneToOne() decorator to declare the one-to-one relationship. user.entity.ts import { Entity, Property, PrimaryKey, OneToOne } from '@mikro-orm/core'; import Address from './address.entity'; @Entity() class User { @PrimaryKey() id: number; @Property({ unique: true }) email: string; @Property() name: string; @Property({ hidden: true }) password: string; @OneToOne({ nullable: true }) address?: Address; } export default User;In our application, we assign just one user to a particular address. Because of that, it is a fitting example of a one-to-one relationship. A thing worth noting is that above, we use the serialization built into MikroORM to hide the password property using the hidden: true option. Instead, we would use the serialization built into NestJS, but unfortunately, that would not work correctly with MikroORM. When creating the user, we don’t need to create the address explicitly. Instead, MikroORM does that for us by default. users.service.ts import { Injectable } from '@nestjs/common'; import { CreateUserDto } from './dto/createUser.dto'; import { InjectRepository } from '@mikro-orm/nestjs'; import { EntityRepository } from '@mikro-orm/core'; import User from './user.entity'; @Injectable() class UsersService { constructor( @InjectRepository(User) private readonly userRepository: EntityRepository, ) {} async create(user: CreateUserDto) { const newUser = await this.userRepository.create(user); await this.userRepository.persistAndFlush(newUser); return newUser; } // ... } export default UsersService; The application we build in this article uses authentication. If you wanto to know more about it, check out API with NestJS #3. Authenticating users with bcrypt, Passport, JWT, and cookies The migration Doing all of the above and running npx mikro-orm migration:create gives us the following migration: Migration20220529210655.ts import { Migration } from '@mikro-orm/migrations'; export class Migration20220529210655 extends Migration { async up(): Promise { this.addSql('create table "address" ("id" serial primary key, "street" varchar(255) not null, "city" varchar(255) not null, "country" varchar(255) not null);'); this.addSql('alter table "user" add column "address_id" int null;'); this.addSql('alter table "user" add constraint "user_address_id_foreign" foreign key ("address_id") references "address" ("id") on update cascade on delete set null;'); this.addSql('alter table "user" add constraint "user_address_id_unique" unique ("address_id");'); } async down(): Promise { this.addSql('alter table "user" drop constraint "user_address_id_foreign";'); this.addSql('drop table if exists "address" cascade;'); this.addSql('alter table "user" drop constraint "user_address_id_unique";'); this.addSql('alter table "user" drop column "address_id";'); } } If you want to know more about generating migrations with MikroORM, check out API with NestJS #62. Introduction to MikroORM with PostgreSQL The crucial thing to notice is that MikroORM adds the address_id column to the user table and makes sure it references the address table. MikroORM also adds a unique constraint to the address_id column. Doing that makes sure the relationship is one-to-one because only one user can refer to a particular address. Trying to relate a second user to the same address would result in an error. Fetching the related entities We need to explicitly tell MikroORM to fetch the address of a given user if we want to do that. users.service.ts import { Injectable } from '@nestjs/common'; import { UserNotFoundException } from './exceptions/userNotFound.exception'; import { InjectRepository } from '@mikro-orm/nestjs'; import { EntityRepository } from '@mikro-orm/core'; import User from './user.entity'; @Injectable() class UsersService { constructor( @InjectRepository(User) private readonly userRepository: EntityRepository, ) {} async getByEmail(email: string) { const user = await this.userRepository.findOne( { email, }, { populate: ['address'], }, ); if (!user) { throw new UserNotFoundException(); } return user; } // ... } export default UsersService;Thanks to doing the above, whenever we call the getByEmail function, we get both the user and the address. One-To-Many and Many-To-One We need the one-to-many relationship when we want a row from the first table to link to multiple rows in the second table. With this approach, the rows from the second table can be linked to just one row from the first table, though. A great example is a post entity and an author. The user can be an author of multiple posts, but a particular post can only have one author. We need to use the ManyToOne() decorator to create a relationship like that. post.entity.ts import { Entity, Property, PrimaryKey, ManyToOne } from '@mikro-orm/core'; import User from '../users/user.entity'; @Entity() class PostEntity { @PrimaryKey() id: number; @Property() title: string; @Property() content: string; @ManyToOne() author: User; } export default PostEntity;To define an author of a post, we need to make sure that we pass it to our PostsService. posts.controller.ts import { Body, Controller, Post, UseGuards, Req, } from '@nestjs/common'; import { PostsService } from './posts.service'; import CreatePostDto from './dto/createPost.dto'; import JwtAuthenticationGuard from '../authentication/jwt-authentication.guard'; import RequestWithUser from '../authentication/requestWithUser.interface'; @Controller('posts') export default class PostsController { constructor(private readonly postsService: PostsService) {} @Post() @UseGuards(JwtAuthenticationGuard) async createPost( @Body() post: CreatePostDto, @Req() request: RequestWithUser, ) { return this.postsService.createPost(post, request.user); } // ... }We also need to create the data for the post entity properly and add the information about the author. posts.service.ts import { Injectable } from '@nestjs/common'; import { InjectRepository } from '@mikro-orm/nestjs'; import { EntityRepository } from '@mikro-orm/core'; import PostEntity from './post.entity'; import CreatePostDto from './dto/createPost.dto'; import User from '../users/user.entity'; @Injectable() export class PostsService { constructor( @InjectRepository(PostEntity) private readonly postRepository: EntityRepository, ) {} async createPost(post: CreatePostDto, user: User) { const postData = { ...post, author: user, }; const newPost = await this.postRepository.create(postData); await this.postRepository.persistAndFlush(newPost); return newPost; } // ... }Thanks to the above, whenever an authenticated user creates a post, we store the information about the user. The migration Thanks to creating the above entities, MikroORM generates the following migration: Migration20220529225632.ts import { Migration } from '@mikro-orm/migrations'; export class Migration20220529225632 extends Migration { async up(): Promise { this.addSql('alter table "post_entity" add column "author_id" int not null;'); this.addSql('alter table "post_entity" add constraint "post_entity_author_id_foreign" foreign key ("author_id") references "user" ("id") on update cascade;'); } async down(): Promise { this.addSql('alter table "post_entity" drop constraint "post_entity_author_id_foreign";'); this.addSql('alter table "post_entity" drop column "author_id";'); } }Like before, MikroORM added the author_id column to the post_entity table that references the user. It is crucial to notice that the above migration does not make the author_id unique in contrast to the one-to-one relationship. Thanks to that, a particular user can be the author of multiple posts. Fetching the related entities MikroORM, by default, does not query the details of the related entities. We can alter this behavior by telling MikroORM explicitly to populate the additional information. posts.service.ts import { Injectable } from '@nestjs/common'; import { InjectRepository } from '@mikro-orm/nestjs'; import { EntityRepository } from '@mikro-orm/core'; import PostEntity from './post.entity'; @Injectable() export class PostsService { constructor( @InjectRepository(PostEntity) private readonly postRepository: EntityRepository, ) {} getPosts() { return this.postRepository.findAll({ populate: ['author'], }); } // ... } We can go even further and populate the nested properties too. posts.service.ts import { Injectable } from '@nestjs/common'; import { InjectRepository } from '@mikro-orm/nestjs'; import { EntityRepository } from '@mikro-orm/core'; import PostEntity from './post.entity'; @Injectable() export class PostsService { constructor( @InjectRepository(PostEnti