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# Many To Many ## Many To Many Many to Many 会在两个 model 中添加一张连接表。 例如,您的应用包含了 user 和 language,且一个 user 可以说多种 language,多个 user 也可以说一种 language。 ```go // User 拥有并属于多种 language,`user_languages` 是连接表 type User struct { gorm.Model Languages []Language `gorm:"many2many:user_languages;"` } type Language struct { gorm.Model Name string } ``` 当使用 GORM 的 `AutoMigrate` 为 `User` 创建表时,GORM 会自动创建连接表 ## 反向引用 ```go // User 拥有并属于多种 language,`user_languages` 是连接表 type User struct { gorm.Model Languages []*Language `gorm:"many2many:user_languages;"` } type Language struct { gorm.Model Name string Users []*User `gorm:"many2many:user_languages;"` } ``` ## 重写外键 对于 `many2many` 关系,连接表会同时拥有两个模型的外键,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 ``` 若要重写它们,可以使用标签 `foreignKey`、`reference`、`joinforeignKey`、`joinReferences`。当然,您不需要使用全部的标签,你可以仅使用其中的一个重写部分的外键、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 关系 ```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 ``` ## 预加载 GORM 允许通过 `Preload` 预加载 has many 关联的记录,查看 [预加载](./preload.md) 获取详情 ## Many2Many 的 CURD 查看 [关联模式](./associations.md#Association-Mode) 获取 many2many 相关的用法 ## 自定义连接表 `连接表` 可以是一个全功能的模型,支持 `Soft Delete`、`钩子`、定义更多的字段,就跟其它模型一样。您可以通过 `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 必须定义相关的的外键字段,否则会报错 db.SetupJoinTable(&Person{}, "Addresses", &PersonAddress{}) ``` ## 外键约束 你可以通过标签 `constraint` 并带上 `OnUpdate`、`OnDelete` 实现外键约束,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); ``` ## 复合外键 如果您的模型使用了 [复合主键](../高级主题/composite_primary_key.md),GORM 会默认启用复合外键。 您也可以覆盖默认的外键、指定多个外键,只需用逗号分隔那些键名,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 ``` 还可以查看 [复合主键](../高级主题/composite_primary_key.md)