gojsj2025/backend/docs/gorm/Many To Many.md

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