Belongs To

Gehört zu

Eine belongs to Zuordnung stellt eine Einzel-zu-Eins-Verbindung mit einem anderen Modell her, so dass jede Instanz des deklarierenden Modells zu einer Instanz des anderen Modells gehört.

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` gehört zu `Company`, `CompanyID` ist der Fremdschlüssel
Typ User struct {
  gorm.Model
  Name string
  CompanyID int
  Company Company
}

Typ Company struct {
  ID int
  Name string
}

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

Fremdschlüssel Überschreiben

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"`
  // benutze CompanyRefer als Fremdschlüssel
}

type Company struct {
  ID   int
  Name string
}

Referenz Überschreiben

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"` // benutze Code als Referenz
}

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 mit zugehörigkeit

Please checkout Association Mode for working with belongs to relations

Voraus-Laden

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

Fremdschlüssel Bedingungen

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
}