Golang Customized UnmarshalJSON Use Case https://play.golang.org/p/HwYr3Pm6hg_P

custom_unmarshal_json.go

package main

import (
	"encoding/json"
	"fmt"
)

type Picture string

type Profile struct {
	ID        string  `json:"id"`
	FirstName string  `json:"first_name"`
	LastName  string  `json:"last_name"`
	Name      string  `json:"name"`
	Picture   Picture `json:"picture,omitempty"`
}

func (p *Picture) UnmarshalJSON(b []byte) error {
	var pictureData struct {
		Data struct {
			Url string `json:"url"`
		} `json:"data"`
	}

	if err := json.Unmarshal(b, &pictureData); err != nil {
		return err
	}

	*p = Picture(pictureData.Data.Url)
	return nil
}

func main() {
	var j string = `{
		"name": "Kebin Liu",
		"first_name": "Kebin",
		"last_name": "Liu",
		"picture": {
			"data": {
				"url": "https://platform-lookaside.fbsbx.com/platform/profilepic/?asid= 123456789&height=200&width=200&ext=1560322969&hash=AeQE-Oiml5D2s-2d"
			}
		},
		"id": "123456789"
	}`

	var profile Profile

	json.Unmarshal([]byte(j), &profile)

	fmt.Println(profile)
}