Many To Many

Many To Many

Many to Many は2つのモデル間のjoinを提供します。

例えば、アプリケーションにusersとlanguagesというモデルがある場合を考えます。 userは複数のlanguageをもち、複数のuserがあるlanguageを話せるとします。

// 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
}

Back-Reference

// 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;"`
}

var users []User
language := Language{}

db.First(&language, "id = ?", 111)

db.Model(&language).Related(&users,  "Users")
//// SELECT * FROM "users" INNER JOIN "user_languages" ON "user_languages"."user_id" = "users"."id" WHERE  ("user_languages"."language_id" IN ('111'))

Foreign Keys

type CustomizePerson struct {
  IdPerson string             `gorm:"primary_key:true"`
  Accounts []CustomizeAccount `gorm:"many2many:PersonAccount;association_foreignkey:idAccount;foreignkey:idPerson"`
}

type CustomizeAccount struct {
  IdAccount string `gorm:"primary_key:true"`
  Name      string
}

It will create a many2many relationship for those two structs, and their relations will be saved into join table PersonAccount with foreign keys customize_person_id_person AND customize_account_id_account

Jointable ForeignKey

If you want to change join table’s foreign keys, you could use tag association_jointable_foreignkey, jointable_foreignkey

type CustomizePerson struct {
  IdPerson string             `gorm:"primary_key:true"`
  Accounts []CustomizeAccount `gorm:"many2many:PersonAccount;foreignkey:idPerson;association_foreignkey:idAccount;association_jointable_foreignkey:account_id;jointable_foreignkey:person_id;"`
}

type CustomizeAccount struct {
  IdAccount string `gorm:"primary_key:true"`
  Name      string
}

Self-Referencing

To define a self-referencing many2many relationship, you have to change association’s foreign key in the join table.

to make it different with source’s foreign key, which is generated using struct’s name and its primary key, for example:

type User struct {
  gorm.Model
  Friends []*User `gorm:"many2many:friendships;association_jointable_foreignkey:friend_id"`
}

GORM will create a join table with foreign key user_id and friend_id, and use it to save user’s self-reference relationship.

Then you can operate it like normal relations, e.g:

DB.Preload("Friends").First(&user, "id = ?", 1)

DB.Model(&user).Association("Friends").Append(&User{Name: "friend1"}, &User{Name: "friend2"})

DB.Model(&user).Association("Friends").Delete(&User{Name: "friend2"})

DB.Model(&user).Association("Friends").Replace(&User{Name: "new friend"})

DB.Model(&user).Association("Friends").Clear()

DB.Model(&user).Association("Friends").Count()

Working with Many To Many

db.Model(&user).Related(&languages, "Languages")
//// SELECT * FROM "languages" INNER JOIN "user_languages" ON "user_languages"."language_id" = "languages"."id" WHERE "user_languages"."user_id" = 111

// Preload Languages when query user
db.Preload("Languages").First(&user)

For advanced usage, refer Association Mode