A has one association establishes a one-to-one relationship between two models. Unlike belongs to, the foreign key lives on the associated model’s table, not on the declaring model.
Struct definition
type User struct {
gorm.Model
CreditCard CreditCard
}
type CreditCard struct {
gorm.Model
Number string
UserID uint // foreign key — references users.id
}
has one relationship from the struct field type. The foreign key on CreditCard is UserID by convention (<OwnerType>ID).
Override the foreign key
Use the foreignKey tag to use a different field name on the associated model:
type User struct {
gorm.Model
CreditCard CreditCard `gorm:"foreignKey:UserRefer"`
}
type CreditCard struct {
gorm.Model
Number string
UserRefer uint
}
Override references
By default GORM uses the owner’s primary key as the reference. Point to a different field with references:
type User struct {
gorm.Model
MemberNumber string `gorm:"unique"`
CreditCard CreditCard `gorm:"foreignKey:UserMemberNumber;references:MemberNumber"`
}
type CreditCard struct {
gorm.Model
Number string
UserMemberNumber string
}
Create with a HasOne association
When you create a User, GORM creates the associated CreditCard in the same transaction if the struct is populated:
user := User{
CreditCard: CreditCard{Number: "4111-1111-1111-1111"},
}
db.Create(&user)
// INSERT INTO users DEFAULT VALUES;
// INSERT INTO credit_cards (number, user_id) VALUES ('4111-1111-1111-1111', 1);
Select or Omit:
db.Select("Name").Create(&user) // only save Name field, skip associations
db.Omit("CreditCard").Create(&user) // skip CreditCard association
Query with Preload
Eagerly load the associated record alongside the owner:
var users []User
db.Preload("CreditCard").Find(&users)
// SELECT * FROM users;
// SELECT * FROM credit_cards WHERE user_id IN (1, 2, 3);
var user User
db.Preload("CreditCard").First(&user, 1)
Conditional preload
Filter associated records with a condition string or a function:
// Only preload active credit cards
db.Preload("CreditCard", "number LIKE ?", "4111%").Find(&users)
// Using a function
db.Preload("CreditCard", func(db *gorm.DB) *gorm.DB {
return db.Where("number IS NOT NULL")
}).Find(&users)
Query with Joins
Joins loads the association using a LEFT JOIN in a single SQL statement:
var users []User
db.Joins("CreditCard").Find(&users)
// SELECT users.*, credit_cards.id AS "CreditCard__id", credit_cards.number AS "CreditCard__number"
// FROM users LEFT JOIN credit_cards ON credit_cards.user_id = users.id;
Use InnerJoins instead of Joins when you only want users that have a credit card:db.InnerJoins("CreditCard").Find(&users)
Update a HasOne association
Use Association mode to replace the current associated record:
newCard := CreditCard{Number: "5500-0000-0000-0004"}
db.Model(&user).Association("CreditCard").Replace(&newCard)
db.Model(&user).Association("CreditCard").Clear()
Constraints
Define referential actions on the foreign key constraint:
type User struct {
gorm.Model
CreditCard CreditCard `gorm:"constraint:OnUpdate:CASCADE,OnDelete:SET NULL;"`
}
If you use OnDelete:CASCADE, deleting a User will also delete their CreditCard at the database level. Make sure this matches your application’s intent.
