多对一

原文：https://gorm.io/docs/has_many.html

多对一 Has Many

A has many association sets up a one-to-many connection with another model, unlike has one, the owner could have zero or many instances of models.

​ has many关联设置了一个一对多的连接，与has one不同，拥有者可以有零个或多个模型实例。

For example, if your application includes users and credit card, and each user can have many credit cards.

​ has many关联设置了一个一对多的连接，与has one不同，拥有者可以有零个或多个模型实例。

声明 Declare

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// User有多个CreditCards，UserID是外键 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
}

检索 Retrieve

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// 使用预加载（Eager loading）检索用户列表及其信用卡 Retrieve user list with eager loading credit cards
func GetAll(db *gorm.DB) ([]User, error) {
    var users []User
    err := db.Model(&User{}).Preload("CreditCards").Find(&users).Error
    return users, err
}

覆盖外键 Override 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

​ 要定义一个has many关系，必须存在一个外键。默认的外键名称是拥有者的类型的名称加上其主键字段的名称。

For example, to define a model that belongs to User, the foreign key should be UserID.

​ 例如，要定义一个属于User的模型，外键应该是UserID。

To use another field as foreign key, you can customize it with a foreignKey tag, e.g:

​ 要使用另一个字段作为外键，可以使用带有foreignKey标签的自定义它，例如：

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type User struct {
  gorm.Model
  CreditCards []CreditCard `gorm:"foreignKey:UserRefer"`
}

type CreditCard struct {
  gorm.Model
  Number    string
  UserRefer uint
}

覆盖引用 Override References

GORM usually uses the owner’s primary key as the foreign key’s value, for the 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.

​ GORM通常使用拥有者的主键作为外键的值，对于上面的示例，它是User的ID。

​ 当你将信用卡分配给用户时，GORM会将用户的ID保存到信用卡的UserID字段中。

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

​ 你可以使用带有references标签更改它，例如：

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type User struct {
  gorm.Model
  MemberNumber string
  CreditCards  []CreditCard `gorm:"foreignKey:UserNumber;references:MemberNumber"`
}

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

多态关联 Polymorphism Association

GORM supports polymorphism association for has one and has many, it will save owned entity’s table name into polymorphic type’s field, primary key value into the polymorphic field

​ GORM支持has one和has many的多态关联，它将拥有实体的表名保存到多态类型字段中，并将主键值保存到多态字段中。

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type Dog struct {
  ID   int
  Name string
  Toys []Toy `gorm:"polymorphic:Owner;"`
}

type Toy struct {
  ID        int
  Name      string
  OwnerID   int
  OwnerType string
}

db.Create(&Dog{Name: "dog1", Toys: []Toy{{Name: "toy1"}, {Name: "toy2"}}})
// INSERT INTO `dogs` (`name`) VALUES ("dog1")
// INSERT INTO `toys` (`name`,`owner_id`,`owner_type`) VALUES ("toy1","1","dogs"), ("toy2","1","dogs")

You can change the polymorphic type value with tag polymorphicValue, for example:

​ 你可以使用带有polymorphicValue标签更改多态类型值，例如：

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type Dog struct {
  ID   int
  Name string
  Toys []Toy `gorm:"polymorphic:Owner;polymorphicValue:master"`
}

type Toy struct {
  ID        int
  Name      string
  OwnerID   int
  OwnerType string
}

db.Create(&Dog{Name: "dog1", Toys: []Toy{{Name: "toy1"}, {Name: "toy2"}}})
// INSERT INTO `dogs` (`name`) VALUES ("dog1")
// INSERT INTO `toys` (`name`,`owner_id`,`owner_type`) VALUES ("toy1","1","master"), ("toy2","1","master")

CRUD with Has Many

Please checkout Association Mode for working with has many relations

​ 请查看关联模式以处理具有许多关系的CRUD操作。

Eager Loading

GORM allows eager loading has many associations with Preload, refer Preloading (Eager loading) for details

​ GORM允许使用Preload进行具有许多关联的预加载，请参阅预加载（Eager loading）以获取详细信息。

Self-Referential Has Many

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type User struct {
  gorm.Model
  Name      string
  ManagerID *uint
  Team      []User `gorm:"foreignkey:ManagerID"`
}

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OnUpdate、OnDelete约束的constraint标签在迁移时创建它们，例如：

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type User struct {
  gorm.Model
  CreditCards []CreditCard `gorm:"constraint:OnUpdate:CASCADE,OnDelete:SET NULL;"`
}

type CreditCard struct {
  gorm.Model
  Number string
  UserID uint
}

You are also allowed to delete selected has many associations with Select when deleting, checkout Delete with Select for details

​ 你还可以在删除时选择删除选定的许多关联，请参阅删除带有Select以获取详细信息。