How to Use Multiple JSON Tags for a Single Struct Field in Go?

Multiple JSON Tags for Struct Fields

In a scenario where you receive JSON data from a server and need to unmarshal it into a struct, you might encounter situations where you want to represent the same data with different JSON tags for different purposes.

Customizing JSON Tags

The JSON tag specifies the field name used in the JSON representation. By default, a field's tag is the same as the field name. However, you can customize the tags to change the names used when serializing or deserializing the struct.

Single JSON Tag

Using a single JSON tag is straightforward. Simply specify the desired tag as a string literal:

type Foo struct {
    Name string `json:"name"`
    Age  int    `json:"age"`
}

Multiple JSON Tags

As mentioned in the question, it's not possible to attach multiple JSON tags directly to a single field. However, there's a technique that allows you to work around this limitation.

Struct Casting

The given solution suggests using two structs that have the same field layout. For example:

type Foo struct {
    Name string
    Age  int
}

type Bar struct {
    Name string `json:"employee_name"`
    Age  int    `json:"-"`
}

Then, you can cast the Foo struct to a Bar struct to change the JSON tags. This technique is especially useful when you have a large number of fields:

foo := Foo{Name: "Sam", Age: 20}
bar := (*Bar)(unsafe.Pointer(&foo))

Caution

It's important to note that the second struct should be unexported to prevent it from being accessed outside your current package. This ensures that the casting is only performed as intended and not accidentally misused.

Example

The following code demonstrates the casting technique mentioned above:

package main

import (
    "encoding/json"
    "fmt"
)

type Foo struct {
    Name string
    Age  int
}

type Bar struct {
    Name string `json:"employee_name"`
    Age  int    `json:"-"`
}

func main() {
    foo := Foo{Name: "Sam", Age: 20}
    bar := (*Bar)(unsafe.Pointer(&foo))

    jsonBytes, err := json.Marshal(bar)
    if err != nil {
        fmt.Println(err)
        return
    }

    fmt.Println(string(jsonBytes))
}

This code successfully serializes the Foo struct data using the JSON tags defined in the Bar struct. It produces the following JSON output:

{"employee_name":"Sam"}

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