# Many To Many
## Many To Many
Many to Many 會在兩個 model 中添加一張連接表。
例如，您的應用包含了 user 和 language，且一個 user 可以說多種 language，多個 user 也可以說一種 language。
```go
// User 擁有并屬于多種 language，`user_languages` 是連接表
type User struct {
gorm.Model
Languages []Language `gorm:"many2many:user_languages;"`
}
type Language struct {
gorm.Model
Name string
}
```
當使用 GORM 的 `AutoMigrate` 為 `User` 創建表時，GORM 會自動創建連接表
## 反向引用
```go
// User 擁有并屬于多種 language，`user_languages` 是連接表
type User struct {
gorm.Model
Languages []*Language `gorm:"many2many:user_languages;"`
}
type Language struct {
gorm.Model
Name string
Users []*User `gorm:"many2many:user_languages;"`
}
```
## 重寫外鍵
對于 `many2many` 關系，連接表會同時擁有兩個模型的外鍵，例如：
```go
type User struct {
gorm.Model
Languages []Language `gorm:"many2many:user_languages;"`
}
type Language struct {
gorm.Model
Name string
}
// Join Table: user_languages
// foreign key: user_id, reference: users.id
// foreign key: language_id, reference: languages.id
```
若要重寫它們，可以使用標簽 `foreignKey`、`reference`、`joinforeignKey`、`joinReferences`。當然，您不需要使用全部的標簽，你可以僅使用其中的一個重寫部分的外鍵、引用。
```go
type User struct {
gorm.Model
Profiles []Profile `gorm:"many2many:user_profiles;foreignKey:Refer;joinForeignKey:UserReferID;References:UserRefer;JoinReferences:ProfileRefer"`
Refer uint
}
type Profile struct {
gorm.Model
Name string
UserRefer uint
}
// Which creates join table: user_profiles
// foreign key: user_refer_id, reference: users.refer
// foreign key: profile_refer, reference: profiles.user_refer
```
## 自引用 Many2Many
自引用 many2many 關系
```go
type User struct {
gorm.Model
Friends []*User `gorm:"many2many:user_friends"`
}
// Which creates join table: user_friends
// foreign key: user_id, reference: users.id
// foreign key: friend_id, reference: users.id
```
## 預加載
GORM 允許通過 `Preload` 預加載 has many 關聯的記錄，查看 [預加載](./preload.md) 獲取詳情
## Many2Many 的 CURD
查看 [關聯模式](./associations.md#Association-Mode) 獲取 many2many 相關的用法
## 自定義連接表
`連接表` 可以是一個全功能的模型，支持 `Soft Delete`、`鉤子`、定義更多的字段，就跟其它模型一樣。您可以通過 `SetupJoinTable` 指定它，例如：
```go
type Person struct {
ID int
Name string
Addresses []Address `gorm:"many2many:person_addresses;"`
}
type Address struct {
ID uint
Name string
}
type PersonAddress struct {
PersonID int
AddressID int
CreatedAt time.Time
DeletedAt gorm.DeletedAt
}
func (PersonAddress) BeforeCreate(db *gorm.DB) error {
// ...
}
// PersonAddress 必須定義相關的的外鍵字段，否則會報錯
db.SetupJoinTable(&Person{}, "Addresses", &PersonAddress{})
```
## 外鍵約束
你可以通過標簽 `constraint` 并帶上 `OnUpdate`、`OnDelete` 實現外鍵約束，例如：
```go
type User struct {
gorm.Model
Languages []Language `gorm:"many2many:user_speaks;"`
}
type Language struct {
Code string `gorm:"primarykey"`
Name string
}
// CREATE TABLE `user_speaks` (`user_id` integer,`language_code` text,PRIMARY KEY (`user_id`,`language_code`),CONSTRAINT `fk_user_speaks_user` FOREIGN KEY (`user_id`) REFERENCES `users`(`id`) ON DELETE SET NULL ON UPDATE CASCADE,CONSTRAINT `fk_user_speaks_language` FOREIGN KEY (`language_code`) REFERENCES `languages`(`code`) ON DELETE SET NULL ON UPDATE CASCADE);
```
## 復合外鍵
如果您的模型使用了 [復合主鍵](../高級主題/composite_primary_key.md)，GORM 會默認啟用復合外鍵。
您也可以覆蓋默認的外鍵、指定多個外鍵，只需用逗號分隔那些鍵名，例如：
```go
type Tag struct {
ID uint `gorm:"primaryKey"`
Locale string `gorm:"primaryKey"`
Value string
}
type Blog struct {
ID uint `gorm:"primaryKey"`
Locale string `gorm:"primaryKey"`
Subject string
Body string
Tags []Tag `gorm:"many2many:blog_tags;"`
SharedTags []Tag `gorm:"many2many:shared_blog_tags;ForeignKey:id;References:id"`
LocaleTags []Tag `gorm:"many2many:locale_blog_tags;ForeignKey:id,locale;References:id"`
}
// Join Table: blog_tags
// foreign key: blog_id, reference: blogs.id
// foreign key: blog_locale, reference: blogs.locale
// foreign key: tag_id, reference: tags.id
// foreign key: tag_locale, reference: tags.locale
// Join Table: shared_blog_tags
// foreign key: blog_id, reference: blogs.id
// foreign key: tag_id, reference: tags.id
// Join Table: locale_blog_tags
// foreign key: blog_id, reference: blogs.id
// foreign key: blog_locale, reference: blogs.locale
// foreign key: tag_id, reference: tags.id
```
還可以查看 [復合主鍵](../高級主題/composite_primary_key.md)
