# 领域层之值对象

### 用来描述一个事物的特征，没有唯一标识的对象

 譬如用户的QQ、手机等。其实除了唯一标志，其他任何字段都可以是值对象

#### 值对象有2个重要原则

1、实体只能通过ID（唯一标识） 来判断两者是否相同

2、而值对象。只需根据“值”就能判断两者是否相等

#### 不可变的特性

 修改值对象。必须传入新对象

### 来个示例：

首先这是实体 -- 用户实体

```
type UserModel struct {
   UserId int `gorm:"column:user_id;primary_key;auto_increment" json:"user_id"  xorm:"'user_id'"`
   UserName string `gorm:"column:user_name" json:"user_name" xorm:"'user_name'"`
   Extra *valueobjs.UserExtra   `gorm:"embedded"`//值对象
 
}
```

这是值对象-- 用户值对象，包含了QQ、手机等

```
type UserExtra struct {
   UserPhone string `gorm:"column:user_phone" json:"user_phone" xorm:"'user_phone'"`
   UserQQ string `gorm:"column:user_qq" json:"user_qq" xorm:"'user_qq'"`
   UserCity string `gorm:"column:user_city" json:"user_city" xorm:"'user_city'"`
}
```

在这里面，假设userid 是唯一标识

那么 

user1:=\&UserModel{UserId:101,UserName:"lisi"}

user2:=\&UserModel{UserId:102,UserName:"lisi"}

这两个实体，哪怕都叫lisi。但是ID不同，所以这两个实体是不一样的

但是如果这两个实体的QQ、手机和所在城市都一样，那么可以认为这两个用户实体的“Extra值”是一样的


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