# Many-to-many relations

Many-to-many is a relation where A contains multiple instances of B, and B contain multiple instances of A.
Let's take for example `Question` and `Category` entities.
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(type => 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) |                            |
+-------------+--------------+----------------------------+

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

Example how to save such relation:

```typescript
const category1 = new Category();
category1.name = "animals";
await connection.manager.save(category1);

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

const question = new Question();
question.categories = [category1, category2];
await connection.manager.save(question);
```

With cascades enabled you can save this relation with only one `save` call.

To load question with categories inside you must specify relation in `FindOptions`:
 
```typescript
const questionRepository = connection.getRepository(Question);
const questions = await questionRepository.find({ relations: ["categories"] });
```

Or using `QueryBuilder` you can join them:

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

With eager loading enabled on a relation you don't have to specify relation or join it - it will ALWAYS be loaded automatically.

Relations can be uni-directional and bi-directional. 
Uni-directional are relations with a relation decorator only on one side.
Bi-directional 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(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)
    @JoinTable()
    categories: Category[];
    
}
```

We just made our relation bi-directional. Note, 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 connection
    .getRepository(Category)
    .createQueryBuilder("category")
    .leftJoinAndSelect("category.questions", "question")
    .getMany();
```