Belongs To

A belongs to association sets up a one-to-one connection with another model, such that each instance of the declaring model “belongs to” one instance of the other model.

For example, if your application includes users and companies, and each user can be assigned to exactly one company, the following types represent that relationship. Notice here that, on the User object, there is both a CompanyID as well as a Company. By default, the CompanyID is implicitly used to create a foreign key relationship between the User and Company tables, and thus must be included in the User struct in order to fill the Company inner struct.

// `User` belongs to `Company`, `CompanyID` is the foreign key
type User struct {
  gorm.Model
  Name      string
  CompanyID int
  Company   Company
}
type Company struct {
  ID   int
  Name string
}

Refer to Eager Loading for details on populating the inner struct.

Override Foreign Key

To define a belongs to relationship, the foreign key must exist, the default foreign key uses the owner’s type name plus its primary field name.

For the above example, to define the User model that belongs to Company, the foreign key should be CompanyID by convention

GORM provides a way to customize the foreign key, for example:

type User struct {
  gorm.Model
  Name         string
  CompanyRefer int
  Company      Company `gorm:"foreignKey:CompanyRefer"`
  // use CompanyRefer as foreign key
}
type Company struct {
  ID   int
  Name string
}

Override References

For a belongs to relationship, GORM usually uses the owner’s primary field as the foreign key’s value, for the above example, it is Company‘s field ID.

When you assign a user to a company, GORM will save the company’s ID into the user’s CompanyID field.

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

type User struct {
  gorm.Model
  Name      string
  CompanyID string
  Company   Company `gorm:"references:Code"` // use Code as references
}
type Company struct {
  ID   int
  Code string
  Name string
}

NOTE GORM usually guess the relationship as has one if override foreign key name already exists in owner’s type, we need to specify references in the belongs to relationship.

type User struct {
  gorm.Model
  Name      string
  CompanyID string
  Company   Company `gorm:"references:CompanyID"` // use Company.CompanyID as references
}
type Company struct {
  CompanyID   int
  Code        string
  Name        string
}

CRUD with Belongs To

Please checkout Association Mode for working with belongs to relations

Eager Loading

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

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
  Name      string
  CompanyID int
  Company   Company `gorm:"constraint:OnUpdate:CASCADE,OnDelete:SET NULL;"`
}
type Company struct {
  ID   int
  Name string
}