多对多

多对多关系会在两个模型之间添加一个连接表。

例如，如果您的应用程序包含用户和语言，并且一个用户可以讲多种语言，而多种语言可以由多个用户使用。

// User has and belongs to many languages, `user_languages` is the join table
type User struct {
  gorm.Model
  Languages []Language `gorm:"many2many:user_languages;"`
}

type Language struct {
  gorm.Model
  Name string
}

当使用 GORM AutoMigrate 为 User 创建表时，GORM 会自动创建连接表

反向引用

声明

// User has and belongs to many languages, use `user_languages` as join table
type User struct {
  gorm.Model
  Languages []*Language `gorm:"many2many:user_languages;"`
}

type Language struct {
  gorm.Model
  Name string
  Users []*User `gorm:"many2many:user_languages;"`
}

检索

// Retrieve user list with eager loading languages
func GetAllUsers(db *gorm.DB) ([]User, error) {
  var users []User
  err := db.Model(&User{}).Preload("Languages").Find(&users).Error
  return users, err
}

// Retrieve language list with eager loading users
func GetAllLanguages(db *gorm.DB) ([]Language, error) {
  var languages []Language
  err := db.Model(&Language{}).Preload("Users").Find(&languages).Error
  return languages, err
}

覆盖外键

对于 多对多 关系，连接表拥有引用两个模型的外键，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

要覆盖它们，您可以使用标签 foreignKey、references、joinForeignKey、joinReferences，不必同时使用它们，您可以只使用其中一个来覆盖某些外键/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
某些数据库只允许创建引用具有唯一索引字段的数据库外键，因此如果要在迁移时创建数据库外键，则需要指定 唯一索引 标签

自引用多对多

自引用多对多关系

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 允许使用 Preload 预加载许多关联，有关详细信息，请参阅预加载（预先加载）

使用多对多进行增删改查

请查看关联模式以了解如何使用多对多关系

自定义连接表

JoinTable 可以是一个功能齐全的模型，例如具有 软删除、钩子 支持和更多字段，您可以使用 SetupJoinTable 进行设置，例如

注意
自定义连接表的外键必须是组合主键或组合唯一索引

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 {
  // ...
}

// Change model Person's field Addresses' join table to PersonAddress
// PersonAddress must defined all required foreign keys or it will raise error
err := db.SetupJoinTable(&Person{}, "Addresses", &PersonAddress{})

外键约束

您可以使用标签 constraint 设置 OnUpdate、OnDelete 约束，它将在使用 GORM 迁移时创建，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);

您还可以在删除时使用 Select 删除选定的多对多关系，有关详细信息，请查看使用选择删除

组合外键

如果您正在为模型使用组合主键，GORM 默认情况下将启用组合外键

您可以覆盖默认外键，要指定多个外键，只需用逗号分隔这些键的名称，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"`
}

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

// Join Table: shared_blog_tags
//   foreign key: blog_id, reference: blogs.id
//   foreign key: tag_id, reference: tags.id

另请查看组合主键

多对多

多对多