2.4 KiB
Has Many Has Many A has many association sets up a one-to-many connection with another model, unlike has one, the owner could have zero or many instances of models.
For example, if your application includes users and credit card, and each user can have many credit cards.
Declare // User has many CreditCards, UserID is the foreign key type User struct { gorm.Model CreditCards []CreditCard }
type CreditCard struct { gorm.Model Number string UserID uint } Retrieve // Retrieve user list with eager loading credit cards func GetAll(db *gorm.DB) ([]User, error) { var users []User err := db.Model(&User{}).Preload("CreditCards").Find(&users).Error return users, err } Override Foreign Key To define a has many relationship, a foreign key must exist. The default foreign key’s name is the owner’s type name plus the name of its primary key field
For example, to define a model that belongs to User, the foreign key should be UserID.
To use another field as foreign key, you can customize it with a foreignKey tag, e.g:
type User struct {
gorm.Model
CreditCards []CreditCard gorm:"foreignKey:UserRefer"
}
type CreditCard struct { gorm.Model Number string UserRefer uint } Override References GORM usually uses the owner’s primary key as the foreign key’s value, for the above example, it is the User‘s ID,
When you assign credit cards to a user, GORM will save the user’s ID into credit cards’ UserID field.
You are able to change it with tag references, e.g:
type User struct {
gorm.Model
MemberNumber string
CreditCards []CreditCard gorm:"foreignKey:UserNumber;references:MemberNumber"
}
type CreditCard struct { gorm.Model Number string UserNumber string } CRUD with Has Many Please checkout Association Mode for working with has many relations
Eager Loading GORM allows eager loading has many associations with Preload, refer Preloading (Eager loading) for details
Self-Referential Has Many
type User struct {
gorm.Model
Name string
ManagerID *uint
Team []User gorm:"foreignkey:ManagerID"
}
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
CreditCards []CreditCard gorm:"constraint:OnUpdate:CASCADE,OnDelete:SET NULL;"
}
type CreditCard struct { gorm.Model Number string UserID uint } You are also allowed to delete selected has many associations with Select when deleting, checkout Delete with Select for details