undeexcepturi/docs/versioned_docs/version-4.5/loading-strategies.md

TypeScript ORM for Node.js based on Data Mapper, Unit of Work and Identity Map patterns. Supports MongoDB, MySQL, PostgreSQL and SQLite databases as well as usage with vanilla JavaScript.

---
title: Relationship Loading Strategies
sidebar_label: Loading Strategies
---

> `JOINED` loading strategy is SQL only feature.

Controls how relationships get loaded when querying. By default, populated relationships are loaded via the `select-in` strategy. This strategy issues one additional `SELECT` statement per relation being loaded.

The loading strategy can be specified both at mapping time and when loading entities.

For example, given the following entities:

```typescript
import { Entity, LoadStrategy, OneToMany, ManyToOne } from '@mikro-orm/core';

@Entity()
export class Author {
  @OneToMany(() => Book, b => b.author)
  books = new Collection<Book>(this);
}

@Entity()
export class Book {
  @ManyToOne()
  author: Author;
}
```

The following will issue two SQL statements. One to load the author and another to load all the books belonging to that author:

```typescript
const author = await orm.em.findOne(Author, 1, ['books']);
```

If we update the `Author.books` mapping to the following:

```typescript
import { Entity, LoadStrategy, OneToMany } from '@mikro-orm/core';

@Entity()
export class Author {
  @OneToMany({
    entity: () => Book,
    mappedBy: b => b.author,
    strategy: LoadStrategy.JOINED,
  })
  books = new Collection<Book>(this);
}
```

The following will issue **one** SQL statement:

```typescript
const author = await orm.em.findOne(Author, 1, ['books']);
```

You can also specify the load strategy as needed. This will override whatever strategy is declared in the mapping. This also works for nested populates:

```typescript
// one level
const author = await orm.em.findOne(Author, 1, {
  populate: {
    books: LoadStrategy.JOINED,
  },
});

// two or more levels - use `FindOptions.strategy`
const author = await orm.em.findOne(Author, 1, {
  populate: {
    books: { publisher: true },
  },
  strategy: LoadStrategy.JOINED
});
```

## Changing the loading strategy globally

You can use `loadStrategy` option in the ORM config:

```ts
MikroORM.init({
  loadStrategy: LoadStrategy.JOINED,
});
```

This value will be used as the default, specifying the loading strategy on property level has precedence, as well as specifying it in the `FindOptions`.