多對多

多對多會在兩個模型之間新增一個關聯表格。

例如，如果您的應用程式包含使用者和語言，而且一個使用者可以說多種語言，且許多使用者可以說一種指定的語言。

// 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
}

使用 GORM AutoMigrate 為 User 建立表格時，GORM 會自動建立關聯表格

反向參考

宣告

// 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;"`
}

擷取

// 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
}

覆寫外來鍵

對於 many2many 關係，關聯表格擁有參考兩個模型的外來鍵，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

若要覆寫它們，可以使用標籤 foreignKey、references、joinForeignKey、joinReferences，不必一起使用，您只要使用其中一個來覆寫某些外來鍵/參考即可

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
有些資料庫只允許建立參考具有唯一索引欄位的資料庫外來鍵，因此如果您在移轉時建立資料庫外來鍵，則需要指定 unique index 標籤

自參考多對多

自參考多對多關係

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

熱切載入

GORM 允許熱切載入與 Preload 有許多關聯，請參閱預載入 (熱切載入) 以取得詳細資訊

使用 Many2Many 進行 CRUD

請查看關聯模式以了解如何使用 many2many 關聯

自訂 JoinTable

JoinTable 可以是一個功能齊全的模型，例如具有 Soft Delete、Hooks 支援和更多欄位，您可以使用 SetupJoinTable 進行設定，例如

注意
自訂的關聯表格外來鍵需要是複合主鍵或複合唯一索引

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{})

FOREIGN KEY 約束

您可以使用標籤 constraint 設定 OnUpdate、OnDelete 約束，它會在使用 GORM 遷移時建立，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);

您也可以在刪除時使用 Select 刪除選取的 many2many 關聯，請查看使用 Select 刪除以取得詳細資訊

複合外來鍵

如果您對模型使用複合主鍵，GORM 會預設啟用複合外來鍵

您可以覆寫預設外來鍵，若要指定多個外來鍵，請使用逗號分隔這些鍵的名稱，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

另請查看複合主鍵

多對多

多對多