Go – 访问指针结构体的字段

huangapple
go评论111阅读模式
英文:

Go - Accessing fields of a pointer struct

问题

我有一个包含敏感字段（如密码和电子邮件）的User结构体。对于一个公共的User实例，例如活动页面上的公共RSVP，我希望在我的JSON输出中排除敏感字段的显示，即使它们是空的。

根据这篇文章，我正在使用一个复合结构体来屏蔽不需要的字段。

问题：在我的数据库函数中的rows.Scan过程中，我如何正确访问复合结构体中指针结构体的字段？我得到了恐慌错误，因为找不到字段。

我的常规User结构体：

  1. type User struct {
  2.     ID          int    `json:"id"`
  3.     FirstName   string `json:"firstname"`
  4.     LastName    string `json:"lastname"`
  5.     Registered  int    `json:"registered"`
  6.     Email       string `json:"email"`
  7.     Password    string `json:"password"`
  8.     ProfilePic  string `json:"profilepic"`
  9. }

根据上述文章中的方法，新增加的结构体：

  1. type omit *struct {}
  3. type PublicUser struct {
  4.     *User
  5.     Registered omit `json:"registered,omitempty"`
  6.     Email      omit `json:"email,omitempty"`
  7.     Password   omit `json:"password,omitempty"`
  8. }

我的数据库函数中出现错误的地方：

  1. func getUsers(db *sql.DB) ([]PublicUser, error) {
  3.     query := `SELECT users.id, users.name_first, users.name_last
  4.     FROM users
  5.     ORDER BY users.name_first asc`
  7.     users := []PublicUser{}
  9.     rows, err := db.Query(query, e.ID)
  10.     if err != nil {
  11.         return nil, err
  12.     }
  13.     defer rows.Close()
  15.     for rows.Next() {
  17.         var user PublicUser
  19.         // 错误发生在这里。似乎在PublicUser中找不到这些字段。
  20.         // User的指针是否在我的PublicUser结构体中起作用？
  22.         err := rows.Scan(&user.ID, &user.FirstName, &user.LastName)
  24.         if err != nil {
  25.             return nil, err
  26.         }
  28.         users = append(users, user)
  29.     }
  31.     return users, nil
  32. }

我原始的JSON输出，不使用文章中的方法，只使用我的常规User结构体：

  1. [{
  2.     "ID": 25,
  3.     "FirstName": "Jim",
  4.     "LastName": "Brown",
  5.     "Registered": 0,
  6.     "Email": "",
  7.     "Password": "",
  8.     "ProfilePic": ""
  9. },
  10. …
  11. ]

期望的JSON输出：

  1. [{
  2.     "ID": 25,
  3.     "FirstName": "Jim",
  4.     "LastName": "Brown",
  5.     "ProfilePic": ""
  6. },
  7. …
  8. ]

文章链接：https://attilaolah.eu/2014/09/10/json-and-struct-composition-in-go/

英文:

I have a User struct containing sensitive fields like password and email. For a public instance of User, for example a public RSVP on an event page, I want to exclude sensitive fields from appearing in my JSON output, even if they're blank.

Based on this article, I’m using a composite struct to mask undesired fields.

QUESTION: during rows.Scan in my database func, how do I properly access the fields of the pointer struct within the composite struct? I’m getting panic errors thrown, because fields are not being found.

My regular User struct:

  1. type User struct {
  2. 	ID		    int `json:"id"`
  3. 	FirstName	string `json:"firstname"`
  4. 	LastName	string `json:"lastname"`
  5. 	Registered	int `json:"registered"`
  6. 	Email		string `json:"email"`
  7. 	Password	string `json:"password"`
  8. 	ProfilePic	string `json:"profilepic"`
  9. }

The new additional structs based on the method in the article linked above:

  1. type omit *struct {
  3. }
  6. type PublicUser struct {
  7. 	*User
  8. 	Registered omit `json:”registered,omitempty"`
  9. 	Email omit `json:”email,omitempty"`
  10. 	Password omit `json:"password,omitempty"`
  11. }

My database func, where an error is occuring:

  1. func getUsers(db *sql.DB) ([]PublicUser, error) {
  3. 	query := `SELECT users.id, users.name_first, users.name_last
  4. 	FROM users
  5. 	ORDER BY users.name_first asc`
  7. 	users := []PublicUser{}
  9. 	rows, err := db.Query(query, e.ID)
  10. 	if err != nil {
  11. 		return nil, err
  12. 	}
  13. 	defer rows.Close()
  15. 	for rows.Next() {
  17. 		var user PublicUser
  20. 		// ERROR OCCURS HERE. Seems like these fields cannot be found in PublicUser.
  21. 		// Is the pointer to User working within my PublicUser struct?
  23. 		err := rows.Scan(&user.ID, &user.FirstName, &user.LastName)
  27. 		if err != nil {
  28. 			return nil, err
  29. 		}
  31. 		users = append(users, user)
  32. 	}
  34. 	return users, nil
  36. }

My original JSON output, not using the articles' method; only using my regular User struct:

  1. [{
  3.     "ID": 25,
  4.     "FirstName": "Jim",
  5.     "LastName": "Brown",
  6.     "Registered": 0,
  7.     "Email": "",
  8.     "Password": "",
  9.     "ProfilePic": ""
  11. },
  12. …
  13. ]

Desired JSON output:

  1. [{
  3.     "ID": 25,
  4.     "FirstName": "Jim",
  5.     "LastName": "Brown",
  6.     "ProfilePic": ""
  8. },
  9. …
  10. ]

答案1

得分: 3

问题在于当你在这里初始化变量时：

  1. var user PublicUser

user 的所有字段都会取它们的“零”值。

由于你嵌入了一个指针，并且指针的零值是 nil，所以你不能在不出错的情况下使用该指针。

为了使其工作，你应该像这样初始化 user：

  1. user = PublicUser{ User: &User{} }

（或者不将其声明为指针）

在这里的 playground 中可以看到问题，并且按照上述描述的方式初始化变量使其工作。

https://play.golang.org/p/fXwvATUm_L

英文:

Problem there is that when you initialize the variable here:

  1. var user PublicUser

All the fields for user take their "zero" values.

Since you are embedding a pointer, and zero value for pointers is nil, you can't use that pointer without getting an error.

In order for this to work, you should initialize user like this:

  1. user = PublicUser{ User: &User{} }

(or don't declare it as a pointer)

See a playground here showing the issue and then initializing the variable as described above for it to work.

https://play.golang.org/p/fXwvATUm_L

huangapple
  • 本文由 发表于 2017年9月17日 20:07:52
  • 转载请务必保留本文链接：https://go.coder-hub.com/46263893.html
  • go
  • struct
匿名

发表评论

匿名网友

:?: :razz: :sad: :evil: :!: :smile: :oops: :grin: :eek: :shock: :???: :cool: :lol: :mad: :twisted: :roll: :wink: :idea: :arrow: :neutral: :cry: :mrgreen:

确定