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。

// 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 AutoMigrateUser 创建表时,GORM 会自动创建连接表

反向引用

// 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:

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 foreignKeyreferencesjoinforeignKeyjoinReferences。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。

type User struct {
gorm.Model
Profiles []Profile `gorm:"many2many:user_profiles;foreignKey:Refer;joinForeignKey:UserReferID;References:UserRefer;joinReferences:ProfileRefer"`
Refer uint `gorm:"index:,unique"`
}

type Profile struct {
gorm.Model
Name string
UserRefer uint `gorm:"index:,unique"`
}

// Which creates join table: user_profiles
// foreign key: user_refer_id, reference: users.refer
// foreign key: profile_refer, reference: profiles.user_refer

注意: 某些数据库只允许在唯一索引字段上创建外键,如果您在迁移时会创建外键,则需要指定 unique index 标签。

自引用 Many2Many

自引用 many2many 关系

type User struct {
gorm.Model
Friends []*User `gorm:"many2many:user_friends"`
}

// 会创建连接表:user_friends
// foreign key: user_id, reference: users.id
// foreign key: friend_id, reference: users.id

预加载

GORM 可以通过 Preload 预加载 has many 关联的记录,查看 预加载 获取详情

Many2Many 的 CURD

查看 关联模式 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、更多的字段,就跟其它模型一样。您可以通过 SetupJoinTable 指定它,例如:

注意: 自定义连接表要求外键是复合主键或复合唯一索引

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 `gorm:"primaryKey"`
AddressID int `gorm:"primaryKey"`
CreatedAt time.Time
DeletedAt gorm.DeletedAt
}

func (PersonAddress) BeforeCreate(db *gorm.DB) error {
// ...
}

// 修改 Person 的 Addresses 字段的连接表为 PersonAddress
// PersonAddress 必须定义好所需的外键,否则会报错
err := db.SetupJoinTable(&Person{}, "Addresses", &PersonAddress{})

外键约束

你可以通过为标签 constraint 配置 OnUpdateOnDelete 实现外键约束,在使用 GORM 进行迁移时它会被创建,例如:

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);

你也可以在删除记录时通过 Select 来删除 many2many 关系的记录,查看 Delete with Select 获取详情

复合外键

如果您的模型使用了 复合主键,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:

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;"`
LocaleTags []Tag `gorm:"many2many:locale_blog_tags;ForeignKey:id,locale;References:id"`
SharedTags []Tag `gorm:"many2many:shared_blog_tags;ForeignKey:id;References:id"`
}

// 连接表: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

// 连接表:locale_blog_tags
// foreign key: blog_id, reference: blogs.id
// foreign key: blog_locale, reference: blogs.locale
// foreign key: tag_id, reference: tags.id

// 连接表:shared_blog_tags
// foreign key: blog_id, reference: blogs.id
// foreign key: tag_id, reference: tags.id

查看 复合主键 获取详情

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