Eager and Lazy Relations

Eager relations

Eager relations are loaded automatically each time you load entities from the database. For example:

import { Entity, PrimaryGeneratedColumn, Column, ManyToMany } from "typeorm"
import { Question } from "./Question"

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

    @Column()
    name: string

    @ManyToMany((type) => Question, (question) => question.categories)
    questions: Question[]
}

import {
    Entity,
    PrimaryGeneratedColumn,
    Column,
    ManyToMany,
    JoinTable,
} from "typeorm"
import { Category } from "./Category"

@Entity()
export class Question {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    title: string

    @Column()
    text: string

    @ManyToMany((type) => Category, (category) => category.questions, {
        eager: true,
    })
    @JoinTable()
    categories: Category[]
}

Now when you load questions you don't need to join or specify relations you want to load. They will be loaded automatically:

const questionRepository = dataSource.getRepository(Question)

// questions will be loaded with its categories
const questions = await questionRepository.find()

Eager relations only work when you use find* methods. If you use QueryBuilder eager relations are disabled and have to use leftJoinAndSelect to load the relation. Eager relations can only be used on one side of the relationship, using eager: true on both sides of relationship is disallowed.

Lazy relations

Entities in lazy relations are loaded once you access them. Such relations must have Promise as type - you store your value in a promise, and when you load them a promise is returned as well. Example:

import { Entity, PrimaryGeneratedColumn, Column, ManyToMany } from "typeorm"
import { Question } from "./Question"

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

    @Column()
    name: string

    @ManyToMany((type) => Question, (question) => question.categories)
    questions: Promise<Question[]>
}

import {
    Entity,
    PrimaryGeneratedColumn,
    Column,
    ManyToMany,
    JoinTable,
} from "typeorm"
import { Category } from "./Category"

@Entity()
export class Question {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    title: string

    @Column()
    text: string

    @ManyToMany((type) => Category, (category) => category.questions)
    @JoinTable()
    categories: Promise<Category[]>
}

categories is a Promise. It means it is lazy and it can store only a promise with a value inside. Example how to save such relation:

const category1 = new Category()
category1.name = "animals"
await dataSource.manager.save(category1)

const category2 = new Category()
category2.name = "zoo"
await dataSource.manager.save(category2)

const question = new Question()
question.categories = Promise.resolve([category1, category2])
await dataSource.manager.save(question)

Example how to load objects inside lazy relations:

const [question] = await dataSource.getRepository(Question).find()
const categories = await question.categories
// you'll have all question's categories inside "categories" variable now

Note: if you come from other languages (Java, PHP, etc.) and are used to using lazy relations everywhere - be careful. Those languages aren't asynchronous, and lazy loading is achieved in a different way, without the use of promises. In JavaScript and Node.JS, you have to use promises if you want to have lazy-loaded relations. This is a non-standard technique and considered experimental in TypeORM.