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Belongs To 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 int
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 }