How to modify tag in golang

Tag in Golang can provide meta-information for structure fields, which can be used for serialization, deserialization, ORM mapping and other scenarios. In actual projects, when requirements change, you may need to change the key value in the tag. This article will introduce how to modify the tag.

1. Immutability

The tag in Golang is immutable, because the tag is written after the type of the structure field and is part of the type definition. If the tag can be changed at will, then The definition of this type will also change, which will break Go's type safety.

2. Solution

Since tag is immutable, how to modify tag? In fact, it can be achieved through reflection.

Assume that there is the following structure definition:

type User struct {
    Id       int64     `json:"user_id" db:"id"`
    Username string    `json:"-" db:"username"`
    Password string    `json:"-" db:"password"`
}

Now we want to modify the db in the tag to db_new, which can be achieved by the following method:

reflect.StructTag(`json:"user_id" db_new:"id"`).Get("db")

## in the code #reflect.StructTag() converts a string into a structure tag type, and the .Get() method can obtain the value of the specified key in the structure tag.

Let’s take a look at the complete code to modify the tag:

package main

import (
    "fmt"
    "reflect"
)

func main() {
    type User struct {
        Id       int64     `json:"user_id" db:"id"`
        Username string    `json:"-" db:"username"`
        Password string    `json:"-" db:"password"`
    }

    user := User{Id: 1, Username: "foo", Password: "bar"}

    field := reflect.TypeOf(user).Field(0)
    tag := field.Tag
    fmt.Println(tag.Get("db"))

    newTag := reflect.StructTag(`json:"user_id" db_new:"id"`)
    newField := reflect.StructField{
        Name:      field.Name,
        Type:      field.Type,
        Tag:       newTag,
        Anonymous: field.Anonymous,
    }
    newType := reflect.StructOf([]reflect.StructField{newField})

    newUser := reflect.New(newType).Elem()
    newUser.Field(0).SetInt(user.Id)

    newTagValue := newUser.Type().Field(0).Tag.Get("db_new")
    fmt.Println(newTagValue)
}

In the above code, we first obtain the tag of the structure field

Id and output its db value. Then use the reflect.StructTag() method to convert the string json:"user_id" db_new:"id" into the structure tag type.

Next, we use

reflect.StructField{} to create a new structure field, specify the new tag value, and some attributes of the original field, such as field name, type, whether it is anonymous, etc. . Then use reflect.StructOf() to create a new structure type.

Finally, we use

reflect.New() to create a new structure instance, copy the value of the Id field in the original structure instance to the new instance, use reflect.Type().Field(0).Tag.Get() Get the db_new value in the new instance.

Run the code, the output is as follows:

id
db_new:"id"

3. Notes

When modifying tags in actual development, you need to pay attention to the following points:

New structure types need to be created through reflection. Compared with directly modifying tags, this will cause additional performance overhead;
• After modifying the tag value, the original ORM mapping may be affected, so it needs to be ensured Modifying the tag value will not cause other functions to malfunction;
• tag can specify multiple key values ​​in the form of key:value, so special attention is required when modifying it.

4. Summary

The above are the methods and precautions for modifying tags in Golang. Generally speaking, it is not recommended to modify tags unless absolutely necessary. In actual projects, you can add a new tag to meet the needs of changing requirements while retaining the invariance of the original tag.

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