属于关系

原文：https://gorm.io/docs/belongs_to.html

属于关系 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.

​ 一个belongs to关联设置了一个模型与另一个模型之间的一对一连接，使得声明模型的每个实例“属于”另一个模型的一个实例。

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.

​ 例如，如果你的应用包括用户和公司，并且每个用户可以被分配给一个公司，那么以下类型表示这种关系。请注意，在User对象上，有一个CompanyID以及一个Company。默认情况下，CompanyID隐式地用于创建User和Company表之间的外键关系，因此必须包含在User结构体中才能填充Company内部结构体。

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// `User`属于`Company`，`CompanyID`是外键 `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.

​ 有关填充内部结构的详细信息，请参阅预加载（预加载）。

覆盖外键 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.

​ 要定义一个属于关系，外键必须存在，默认的外键使用拥有者的类型名称加上其主字段名称。

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

​ 要定义一个属于关系，外键必须存在，默认的外键使用拥有者的类型名称加上其主字段名称。

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

​ GORM提供了一个自定义外键的方法，例如：

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

type Company struct {
  ID   int
  Name string
}

覆盖引用 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.

​ 对于一个属于关系，GORM通常使用拥有者的主字段作为外键的值，对于上面的示例，它是Company的字段ID。

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

​ 当你将用户分配给一家公司时，GORM将公司的ID保存到用户的CompanyID字段中。

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

​ 你可以使用标签references更改它，例如：

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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.

注意 GORM通常在拥有者的类型中已经存在覆盖外键名称的关系时猜测关系为has one，我们需要在belongs to关系中指定references。

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type User struct {
  gorm.Model
  Name      string
  CompanyID string
  Company   Company `gorm:"references:CompanyID"` // 使用Company.CompanyID作为引用 use Company.CompanyID as references
}

type Company struct {
  CompanyID   int
  Code        string
  Name        string
}

与属于关系的CRUD操作 CRUD with Belongs To

Please checkout Association Mode for working with belongs to relations

​ 请查看关联模式以处理属于关系

预加载（预加载） Eager Loading

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

​ GORM允许通过Preload或Joins预加载属于关系，请参阅预加载（预加载）以获取详细信息

FOREIGN KEY约束 FOREIGN KEY Constraints

You can setup OnUpdate, OnDelete constraints with tag constraint, it will be created when migrating with GORM, for example:

​ 你可以使用带有标签constraint的OnUpdate和OnDelete约束来设置OnUpdate、OnDelete约束，当使用GORM进行迁移时，它将被创建，例如：

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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
}