Has Many

Has Many

A has many association also 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.

// 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
}

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 (e.g. UserID, CardID, etc).

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
}

Association ForeignKey

GORM usually uses the owner’s primary key as the foreign key’s value, for 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 association_foreignkey, e.g:

type User struct {
  gorm.Model
  MemberNumber string
  CreditCards  []CreditCard `gorm:"foreignkey:UserMemberNumber;association_foreignkey:MemberNumber"`
}

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

Polymorphism Association

GORM 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 Many

You could find has many associations with Related

db.Model(&user).Related(&emails)
//// SELECT * FROM emails WHERE user_id = 111; // 111 is user's primary key

For advanced usage, refer to Association Mode