> For the complete documentation index, see [llms.txt](https://orkhan.gitbook.io/typeorm/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://orkhan.gitbook.io/typeorm/docs/docs/relations/5-eager-and-lazy-relations.md).

# Eager and Lazy Relations

## Eager relations

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

```typescript
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[]
}
```

```typescript
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:

```typescript
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.

By default, eager relations are loaded using `LEFT JOIN`. If the relation is also marked `nullable: false` (and it owns the join column, i.e. `ManyToOne` or owning `OneToOne`), TypeORM uses `INNER JOIN` instead, which can produce more efficient query plans.

### Relation load strategy

By default, eager relations are loaded by adding SQL JOINs to the main query (`"join"` strategy). If you are loading too much data with nested joins, you can switch to the `"query"` strategy, which loads relations via separate database queries instead:

```typescript
// per-query
const questions = await questionRepository.find({
    relationLoadStrategy: "query",
})

// or set as the default for the entire DataSource
const dataSource = new DataSource({
    // ...
    relationLoadStrategy: "query",
})
```

You can also control whether eager relations are loaded at all using `loadEagerRelations`:

```typescript
// disable eager relation loading entirely
const questions = await questionRepository.find({
    loadEagerRelations: false,
})

// load explicit relations only, suppress nested eager relations
const questions = await questionRepository.find({
    relations: { categories: true },
    loadEagerRelations: false,
})
```

## 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:

```typescript
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[]>
}
```

```typescript
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:

```typescript
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:

```typescript
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.