Many To Many
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
Declare
// 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;"`
}
Retrieve
// 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
}
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, references, 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: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
NOTE:
Some databases only allow create database foreign keys that reference on a field having unique index, so you need to specify the unique index tag if you are creating database foreign keys when migrating

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 more fields, you can set it up with SetupJoinTable, for example:

NOTE:
Customized join table’s foreign keys required to be composited primary keys or composited unique index

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 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);
You are also allowed to delete selected many2many relations with Select when deleting, checkout Delete with Select for details

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