En a un (has one)

Une association en a un met en place une relation un à un avec un autre modèle, mais avec une sémantique quelque peu différente (et des conséquences). Cette association indique que chaque instance d'un modèle contient ou possède une instance d'un autre modèle.

Par exemple, si votre application contient des utilisateurs et des cartes de crédit, et que chaque utilisateur peu avoir qu'une seule carte de crédit.

Déclarer

// User has one CreditCard, UserID is the foreign key
type User struct {
  gorm.Model
  CreditCard CreditCard
}

type CreditCard struct {
  gorm.Model
  Number string
  UserID uint
}

Retrieve

Override Foreign Key

The field's name is usually generated with has one model's type plus its primary key, for the above example it is UserID.

When you give a credit card to the user, it will save the User's ID into its UserID field.

If you want to use another field to save the relationship, you can change it with tag foreignKey, e.g:

type User struct {
  gorm.Model
  CreditCard CreditCard `gorm:"foreignKey:UserName"`
  // use UserName as foreign key
}

type CreditCard struct {
  gorm.Model
  Number   string
  UserName string
}

Override References

By default, the owned entity will save the has one model's primary key into a foreign key, you could change to save another field's value, like using Name for the below example.

You are able to change it with tag references, e.g:

type User struct {
  gorm.Model
  Name       string     `gorm:"index"`
  CreditCard CreditCard `gorm:"foreignKey:UserName;references:Name"`
}

type CreditCard struct {
  gorm.Model
  Number   string
  UserName string
}

CRUD with Has One

Please checkout Association Mode for working with has one relations

Eager Loading

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

Self-Referential Has One

type User struct {
  gorm.Model
  Name      string
  ManagerID *uint
  Manager   *User
}

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
  CreditCard 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 one associations with Select when deleting, checkout Delete with Select for details