How Can I Dynamically Change JSON Tags in Go Structs?

Dynamically Changing Struct's JSON Tag

Struct tags in Go play a crucial role in encoding and decoding JSON data. However, sometimes it becomes necessary to modify these tags dynamically based on certain criteria. In this article, we explore a solution to achieve this using Go reflection and other techniques.

Problem

Consider the following code snippet:

type User struct {
    ID   int64  `json:"id"`
    Name string `json:"first"` // want to change this to `json:"name"`
}

Our goal is to change the JSON tag for the Name field from json:"first" to json:"name" before marshalling the struct to JSON.

Solution: Using an Alias with Custom Tags

With Go 1.8 and above, we can overcome the limitation of immutable struct tags by utilizing an alias with custom tags. Here's how it's done:

type User struct {
    ID   int64  `json:"id"`
    Name string `json:"first"`
}

func (u *User) MarshalJSON() ([]byte, error) {
    type alias struct {
        ID   int64  `json:"id"`
        Name string `json:"name"`
    }
    var a alias = alias(*u)
    return json.Marshal(&a)
}

In the MarshalJSON method, we define an alias type that has the same structure as User but with the desired JSON tags. By assigning u (the original User struct) to a variable of type alias, we effectively change the JSON tags. When we marshal the alias (instead of the original User), the result conforms to the desired JSON format.

Custom Implementation

To dynamically modify the tags for multiple fields, we can employ reflection to iterate through the fields of the User struct and update the tags accordingly. Here's a custom implementation:

import (
    "reflect"
    "strings"
)

func ModifyTags(u *User) {
    value := reflect.ValueOf(u)
    for i := 0; i < value.NumField(); i++ {
        tag := value.Type().Field(i).Tag.Get("json")
        if strings.HasPrefix(tag, "custom:") {
            value.Type().Field(i).Tag.Set("json", strings.TrimPrefix(tag, "custom:"))
        }
    }
}

By calling ModifyTags, we can dynamically modify the tags for the fields that have the "custom:" prefix. This approach allows for flexible tag modification based on specific criteria.

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