Golang处理数据库的nil数据

在用golang获取数据库的数据的时候，难免会遇到可控field。这个时候拿到的数据如果直接用string, time.Time这样的类型来解析的话会遇到panic。

那么如何处理这个问题呢，第一个出现在眼前的办法就是用database/sql。这个包里包含了很多的可以处理可控字段的类型，比如：sql.NullString, sql.NullBool等。所以，model可以用这些类型来定义，如：

package main
import (
    "database/sql"
    "fmt"
    "github.com/go-sql-driver/mysql"
)
type Article struct {
    Id      int            `json:"id"`
    Title   string         `json:"title"`
    PubDate mysql.NullTime `json:"pub_date"`
    Body    sql.NullString `json:"body"`
    User    sql.NullInt64  `json:"user"`
}

这样的定义是可以work的，但是会有一点奇怪：没谁会用数据库的类型来代替平时的类型。所以，我们可以改一种思路。在解析数据库的时候会有专用的数据库类型字段来接收，但是在返回json的时候有专门的model来使用，这个model用的是普通的类型。

所以，写起来是这样的：

var person Person
var personID int64
var password sql.NullString
var lastLogin mysql.NullTime
var isSuperuser sql.NullBool
var userName sql.NullString
var firstName sql.NullString
var lastName sql.NullString
var email sql.NullString
var isStaff sql.NullBool
var isActive sql.NullBool
var dateJoined mysql.NullTime
err = ret.Scan(
	&personID,
    &password,
    &lastLogin,
	&isSuperuser,
	&userName,
	&firstName,
	&lastName,
	&email,
	&isStaff,
	&isActive,
	&dateJoined,
)

上面定义了解析，下面定义model：

type Person struct {
	ID          int64      `json:"id"`
	Password    string     `json:"password"`
	LastLogin   *time.Time `json:"last_login"`
	IsSuperuser bool       `json:"is_superuser"`
	Username    string     `json:"username"`
	FirstName   string     `json:"first_name"`
	LastName    string     `json:"last_name"`
	Email       string     `json:"email"`
	IsStaff     bool       `json:"is_staff"`
	IsActive    bool       `json:"is_active"`
	DateJoined  *time.Time `json:"date_joined"`
}

有了前两部之后，现在可以装配这些数据了：

person.ID = personID
person.Password = If(password.Valid, password.String, "").(string)
if tempTime, ok := If(lastLogin.Valid, lastLogin.Time, nil).(*time.Time); ok {
	person.LastLogin = tempTime
} else {
	person.LastLogin = nil
}
person.IsSuperuser = If(isSuperuser.Valid, isSuperuser.Bool, false).(bool)
person.Username = If(userName.Valid, userName.String, "").(string)
person.FirstName = If(firstName.Valid, firstName.String, "").(string)
person.LastName = If(lastName.Valid, lastName.String, "").(string)
person.Email = If(email.Valid, email.String, "").(string)
person.IsStaff = If(isStaff.Valid, isStaff.Bool, false).(bool)
person.IsActive = If(isActive.Valid, isActive.Bool, false).(bool)
if tempTime, ok := If(dateJoined.Valid, dateJoined.Time, nil).(*time.Time); ok {
	person.DateJoined = tempTime
} else {
	person.DateJoined = nil
}

有一点需要注意的。在golang里类型转换之前需要先做一个type assertion才行，否则报错。而且nil是不能做类型转换的，比如：

if tempTime, ok := If(dateJoined.Valid, dateJoined.Time, nil).(*time.Time); ok {
	person.DateJoined = tempTime
} else {
	person.DateJoined = nil
}

以上是同sqlite做为数据库是遇到的问题。还有一点，sqlite没有处理时间为空的类型，所以上面使用的是mysql的driver里的NullTime，奇怪的是用自定义的NullTime不行。懒得深究了，哪位知道的话希望留言。

下面是全部的app代码：

package main
import (
	"encoding/json"
	"fmt"
	"net/http"
	"time"
	"database/sql"
	"log"
	"github.com/go-sql-driver/mysql"
	"github.com/labstack/echo"
	"github.com/labstack/echo/middleware"
	_ "github.com/mattn/go-sqlite3"
)
func main() {
	// Echo instance
	e := echo.New()
	// Middleware
	e.Use(middleware.Logger())
	e.Use(middleware.Recover())
	// Routes
	e.GET("/", hello)
	// Start server
	e.Logger.Fatal(e.Start(":1323"))
}
func hello(c echo.Context) error {
	ret, err := executeSQL("select id,password,last_login,is_superuser,username,first_name,last_name,email,is_staff,is_active,date_joined from person where id = ?")
	if err != nil {
		return c.JSON(http.StatusOK, map[string]string{"error": "Something went wrong when getting data from db"})
	}
	return c.JSON(http.StatusOK, ret)
}
// Execute sql statement from parameter, which looks like this:
// select a, b, c from some_tabble where id = ?
// Return a map
func executeSQL(sqlStmt string) ([]Person, error) {
	db, err := sql.Open("sqlite3", "./db.sqlite3")
	if err != nil {
		log.Fatal(err)
	}
	defer db.Close()
	stmt, err := db.Prepare(sqlStmt)
	if err != nil {
		log.Fatal(err)
		return nil, err
	}
	defer stmt.Close()
	ret, err := stmt.Query(3)
	if err != nil {
		log.Fatal(err)
		return nil, err
	}
	personList := make([]Person, 0)
	for ret.Next() {
		var person Person
		var personID int64
		var password sql.NullString
		var lastLogin mysql.NullTime
		var isSuperuser sql.NullBool
		var userName sql.NullString
		var firstName sql.NullString
		var lastName sql.NullString
		var email sql.NullString
		var isStaff sql.NullBool
		var isActive sql.NullBool
		var dateJoined mysql.NullTime
		err = ret.Scan(
			&personID,
			&password,
			&lastLogin,
			&isSuperuser,
			&userName,
			&firstName,
			&lastName,
			&email,
			&isStaff,
			&isActive,
			&dateJoined,
		)
		if err != nil {
			log.Fatal(err)
		}
		person.ID = personID
		person.Password = If(password.Valid, password.String, "").(string)
		if tempTime, ok := If(lastLogin.Valid, lastLogin.Time, nil).(*time.Time); ok {
			person.LastLogin = tempTime
		} else {
			person.LastLogin = nil
		}
		person.IsSuperuser = If(isSuperuser.Valid, isSuperuser.Bool, false).(bool)
		person.Username = If(userName.Valid, userName.String, "").(string)
		person.FirstName = If(firstName.Valid, firstName.String, "").(string)
		person.LastName = If(lastName.Valid, lastName.String, "").(string)
		person.Email = If(email.Valid, email.String, "").(string)
		person.IsStaff = If(isStaff.Valid, isStaff.Bool, false).(bool)
		person.IsActive = If(isActive.Valid, isActive.Bool, false).(bool)
		if tempTime, ok := If(dateJoined.Valid, dateJoined.Time, nil).(*time.Time); ok {
			person.DateJoined = tempTime
		} else {
			person.DateJoined = nil
		}
		personList = append(personList, person)
	}
	j, err := json.Marshal(personList)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(j)
	return personList, nil
}