十、自定义数据类型
自定义的数据类型必须实现 Scanner 和 Valuer 接口，以便让 GORM 知道如何将该类型接收、保存到数据库
1. 存储结构体
type Info struct {Status string `json:"status"`Addr string `json:"addr"`Age int `json:"age"`
}// Scan 从数据库中读取出来
func (i *Info) Scan(value interface{}) error {bytes, ok := value.([]byte)if !ok {return errors.New(fmt.Sprint("Failed to unmarshal JSONB value:", value))}info := Info{}err := json.Unmarshal(bytes, &info)*i = inforeturn err
}// Value 存入数据库
func (i Info) Value() (driver.Value, error) {return json.Marshal(i)
}type User struct {ID uintName stringInfo Info `gorm:"type:string"`
}
添加和查询
DB.Create(&User{Name: "枫枫",Info: Info{Status: "牛逼",Addr: "成都市",Age: 21,},
})var user User
DB.Take(&user)
fmt.Println(user)
2. 枚举类型
枚举1.0
很多时候，我们会对一些状态进行判断，而这些状态都是有限的
例如，主机管理中，状态有 Running 运行中， OffLine 离线， Except 异常
如果存储字符串，不仅是浪费空间，每次判断还要多复制很多字符，最主要是后期维护麻烦
type Host struct {ID uintName stringStatus string
}func main() {host := Host{}if host.Status == "Running" {fmt.Println("在线")}if host.Status == "Except" {fmt.Println("异常")}if host.Status == "OffLine" {fmt.Println("离线")}
}
后来，我们知道了用常量存储这些不变的值
type Host struct {ID uintName stringStatus string
}const (Running = "Running"Except = "Except"OffLine = "OffLine"
) func main() {host := Host{}if host.Status == Running {fmt.Println("在线")}if host.Status == Except {fmt.Println("异常")}if host.Status == OffLine {fmt.Println("离线")}
}虽然代码变多了，但是维护方便了
但是数据库中存储的依然是字符串，浪费空间这个问题并没有解决
枚举2.0
于是想到使用数字表示状态
type Host struct {ID uintName stringStatus int
}const (Running = 1Except = 2OffLine = 3
)func main() {host := Host{}if host.Status == Running {fmt.Println("在线")}if host.Status == Except {fmt.Println("异常")}if host.Status == OffLine {fmt.Println("离线")}
}
但是，如果返回数据给前端，前端接收到的状态就是数字，不过问题不大，前端反正都要搞字符映射的
因为要做颜色差异显示
但是这并不是后端偷懒的理由
于是我们想到，在json序列化的时候，根据映射转换回去
type Host struct {ID uint `json:"id"`Name string `json:"name"`Status int `json:"status"`
}func (h Host) MarshalJSON() ([]byte, error) {var status stringswitch h.Status {case Running:status = "Running"case Except:status = "Except"case OffLine :status = "OffLine"}return json.Marshal(&struct {ID uint `json:"id"`Name string `json:"name"`Status string `json:"status"`}{ID: h.ID,Name: h.Name,Status: status,})
}const (Running = 1Except = 2OffLine = 3
)func main() {host := Host{1, "枫枫", Running}data, _ := json.Marshal(host)fmt.Println(string(data)) // {"id":1,"name":"枫枫","status":"Running"}
}
这样写确实可以实现我们的需求，但是根本就不够通用，凡是用到枚举，都得给这个Struct实现MarshalJSON方法
枚举3.0
于是类型别名出来了
type Status intfunc (status Status) MarshalJSON() ([]byte, error) {var str stringswitch status {case Running:str = "Running"case Except:str = "Except"case OffLine:str = "Status"}return json.Marshal(str)
}type Host struct {ID uint `json:"id"`Name string `json:"name"`Status Status `json:"status"`
}const (Running Status = 1Except Status = 2OffLine Status = 3
)func main() {host := Host{1, "枫枫", Running}data, _ := json.Marshal(host)fmt.Println(string(data)) // {"id":1,"name":"枫枫","status":"Running"}
}
嗯，代码简洁了不少，在使用层面已经没有问题了
但是，这个结构体怎么表示数据库中的字段呢？
golang中没有枚举
我们只能自己通过逻辑实现枚举
type Weekday intconst (Sunday Weekday = iota + 1 // EnumIndex = 1Monday // EnumIndex = 2Tuesday // EnumIndex = 3Wednesday // EnumIndex = 4Thursday // EnumIndex = 5Friday // EnumIndex = 6Saturday // EnumIndex = 7
)var WeekStringList = []string{"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"}
var WeekTypeList = []Weekday{Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday}// String 转字符串
func (w Weekday) String() string {return WeekStringList[w-1]
}// MarshalJSON 自定义类型转换为json
func (w Weekday) MarshalJSON() ([]byte, error) {return json.Marshal(w.String())
}// EnumIndex 自定义类型转原始类型
func (w Weekday) EnumIndex() int {return int(w)
}// ParseWeekDay 字符串转自定义类型
func ParseWeekDay(week string) Weekday {for i, i2 := range WeekStringList {if week == i2 {return WeekTypeList[i]}}return Monday
}// ParseIntWeekDay 数字转自定义类型
func ParseIntWeekDay(week int) Weekday {return Weekday(week)
}type DayInfo struct {Weekday Weekday `json:"weekday"`Date time.Time `json:"date"`
}func main() {w := Sundayfmt.Println(w)dayInfo := DayInfo{Weekday: Sunday, Date: time.Now()}data, err := json.Marshal(dayInfo)fmt.Println(string(data), err)week := ParseWeekDay("Sunday")fmt.Println(week)week = ParseIntWeekDay(2)fmt.Println(week)
}
在需要输出的时候（print，json），自定义类型就变成了字符串
从外界接收的数据也能转换为自定义类型，这就是golang中的枚举，假枚举
