# Many-to-many relations

-   [What are many-to-many relations](#what-are-many-to-many-relations)
-   [Saving many-to-many relations](#saving-many-to-many-relations)
-   [Deleting many-to-many relations](#deleting-many-to-many-relations)
-   [Loading many-to-many relations](#loading-many-to-many-relations)
-   [Bi-directional relations](#bi-directional-relations)
-   [Many-to-many relations with custom properties](#many-to-many-relations-with-custom-properties)

## What are many-to-many relations

Many-to-many is a relation where A contains multiple instances of B, and B contains multiple instances of A.
Let's take for example `Question` and `Category` entities.
A question can have multiple categories, and each category can have multiple questions.

```typescript
import { Entity, PrimaryGeneratedColumn, Column } from "typeorm"

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

    @Column()
    name: string
}
```

```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(() => Category)
    @JoinTable()
    categories: Category[]
}
```

`@JoinTable()` is required for `@ManyToMany` relations.
You must put `@JoinTable` on one (owning) side of relation.

This example will produce following tables:

```shell
+-------------+--------------+----------------------------+
|                        category                         |
+-------------+--------------+----------------------------+
| id          | int(11)      | PRIMARY KEY AUTO_INCREMENT |
| name        | varchar(255) |                            |
+-------------+--------------+----------------------------+

+-------------+--------------+----------------------------+
|                        question                         |
+-------------+--------------+----------------------------+
| id          | int(11)      | PRIMARY KEY AUTO_INCREMENT |
| title       | varchar(255) |                            |
| text        | varchar(255) |                            |
+-------------+--------------+----------------------------+

+-------------+--------------+----------------------------+
|              question_categories_category               |
+-------------+--------------+----------------------------+
| questionId  | int(11)      | PRIMARY KEY FOREIGN KEY    |
| categoryId  | int(11)      | PRIMARY KEY FOREIGN KEY    |
+-------------+--------------+----------------------------+
```

## Saving many-to-many relations

With [cascades](./relations.md#cascades) enabled, you can save this relation with only one `save` call.

```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.title = "dogs"
question.text = "who let the dogs out?"
question.categories = [category1, category2]
await dataSource.manager.save(question)
```

## Deleting many-to-many relations

With [cascades](./relations.md#cascades) enabled, you can delete this relation with only one `save` call.

To delete a many-to-many relationship between two records, remove it from the corresponding field and save the record.

```typescript
const question = await dataSource.getRepository(Question).findOne({
    relations: {
        categories: true,
    },
    where: { id: 1 }
})
question.categories = question.categories.filter((category) => {
    return category.id !== categoryToRemove.id
})
await dataSource.manager.save(question)
```

This will only remove the record in the join table. The `question` and `categoryToRemove` records will still exist.

## Soft Deleting a relationship with cascade

This example shows how the cascading soft delete behaves:

```typescript
const category1 = new Category()
category1.name = "animals"

const category2 = new Category()
category2.name = "zoo"

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

await dataSource.manager.softRemove(newQuestion)
```

In this example we did not call save or softRemove for category1 and category2, but they will be automatically saved and soft-deleted when the cascade of relation options is set to true like this:

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

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

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

## Loading many-to-many relations

To load questions with categories inside you must specify the relation in `FindOptions`:

```typescript
const questionRepository = dataSource.getRepository(Question)
const questions = await questionRepository.find({
    relations: {
        categories: true,
    },
})
```

Or using `QueryBuilder` you can join them:

```typescript
const questions = await dataSource
    .getRepository(Question)
    .createQueryBuilder("question")
    .leftJoinAndSelect("question.categories", "category")
    .getMany()
```

With eager loading enabled on a relation, you don't have to specify relations in the find command as it will ALWAYS be loaded automatically. If you use QueryBuilder eager relations are disabled, you have to use `leftJoinAndSelect` to load the relation.

## Bi-directional relations

Relations can be uni-directional and bi-directional.
Uni-directional relations are relations with a relation decorator only on one side.
Bi-directional relations are relations with decorators on both sides of a relation.

We just created a uni-directional relation. Let's make it bi-directional:

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

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

    @Column()
    name: string

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

We just made our relation bi-directional. Note that the inverse relation does not have a `@JoinTable`.
`@JoinTable` must be only on one side of the relation.

Bi-directional relations allow you to join relations from both sides using `QueryBuilder`:

```typescript
const categoriesWithQuestions = await dataSource
    .getRepository(Category)
    .createQueryBuilder("category")
    .leftJoinAndSelect("category.questions", "question")
    .getMany()
```

## Many-to-many relations with custom properties

In case you need to have additional properties in your many-to-many relationship, you have to create a new entity yourself.
For example, if you would like entities `Question` and `Category` to have a many-to-many relationship with an additional `order` column, then you need to create an entity `QuestionToCategory` with two `ManyToOne` relations pointing in both directions and with custom columns in it:

```typescript
import { Entity, Column, ManyToOne, PrimaryGeneratedColumn } from "typeorm"
import { Question } from "./question"
import { Category } from "./category"

@Entity()
export class QuestionToCategory {
    @PrimaryGeneratedColumn()
    public questionToCategoryId: number

    @Column()
    public questionId: number

    @Column()
    public categoryId: number

    @Column()
    public order: number

    @ManyToOne(() => Question, (question) => question.questionToCategories)
    public question: Question

    @ManyToOne(() => Category, (category) => category.questionToCategories)
    public category: Category
}
```

Additionally you will have to add a relationship like the following to `Question` and `Category`:

```typescript
// category.ts
...
@OneToMany(() => QuestionToCategory, questionToCategory => questionToCategory.category)
public questionToCategories: QuestionToCategory[];

// question.ts
...
@OneToMany(() => QuestionToCategory, questionToCategory => questionToCategory.question)
public questionToCategories: QuestionToCategory[];
```
