Has One

Has One

A has one association also sets up a one-to-one connection with another model, but with somewhat different semantics (and consequences). This association indicates that each instance of a model contains or possesses one instance of another model.

For example, if your application includes users and credit cards, and each user
can only have one credit card.

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

type User struct {
  gorm.Model
  CreditCard   CreditCard
}

Foreign Key

For a has one relationship, a foreign key field must also exist, the owned
will save the primary key of the model belongs to it into this field.

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, its 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 CreditCard struct {
  gorm.Model
  Number   string
  UserName string
}

type User struct {
  gorm.Model
  CreditCard CreditCard `gorm:"foreignkey:UserName"`
}

Association ForeignKey

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

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

type User struct {
  gorm.Model
  Name       `sql:"index"`
  CreditCard CreditCard `gorm:"foreignkey:uid;association_foreignkey:name"`
}

Polymorphism Association

Supports polymorphic has many and has one associations.

type Cat struct {
  ID    int
  Name  string
  Toy   Toy `gorm:"polymorphic:Owner;"`
}

type Dog struct {
  ID   int
  Name string
  Toy  Toy `gorm:"polymorphic:Owner;"`
}

type Toy struct {
  ID        int
  Name      string
  OwnerID   int
  OwnerType string
}

Note: polymorphic belongs-to and many-to-many are explicitly NOT supported, and will throw errors.

Working with Has One

You could find has one associations with Related

var card CreditCard
db.Model(&user).Related(&card, "CreditCard")
//// SELECT * FROM credit_cards WHERE user_id = 123; // 123 is user's primary key
// CreditCard is user's field name, it means get user's CreditCard relations and fill it into variable card
// If the field name is same as the variable's type name, like above example, it could be omitted, like:
db.Model(&user).Related(&card)

For advanced usage, refer Association Mode