Many To Many

Many to Many add a join table between two models.

For example, if your application includes users and languages, and a user can speak many languages, and many users can speak a specified language.

// 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
}

When using GORM AutoMigrate to create a table for User, GORM will create the join table automatically

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;"`
}

Override Foreign Key

For a many2many relationship, the join table owns the foreign key which references two models, for example:

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

To override them, you can use tag foreignKey, reference, joinForeignKey, joinReferences, not necessary to use them together, you can just use one of them to override some foreign keys/references

type User struct {
  gorm.Model
  Profiles []Profile `gorm:"many2many:user_profiles;foreignKey:Refer;joinForeignKey:UserReferID;References:UserRefer;JoinReferences:UserRefer"`
  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

Self-Referential Many2Many

Self-referencing many2many relationship

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

Eager Loading

GORM allows eager loading has many associations with Preload, refer Preloading (Eager loading) for details

CRUD with Many2Many

Please checkout Association Mode for working with many2many relations

Customize JoinTable

JoinTable can be a full-featured model, like having Soft Delete，Hooks supports, and define more fields, you can setup it with SetupJoinTable, for example:

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 {
  // ...
}

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

FOREIGN KEY Constraints

You can setup OnUpdate, OnDelete constraints with tag constraint, it will be created when migrating with GORM, for example:

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);

Composite Foreign Keys

If you are using Composite Primary Keys for your models, GORM will enable composite foreign keys by default

You are allowed to override the default foreign keys, to specify multiple foreign keys, just separate those keys’ name by commas, for example:

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

Also check out Composite Primary Keys