How to extract data from map[string]interface{}? - go

Question

I am sending the data to the API like following:
{"after": {"amount": 811,"id":123,"status":"Hi"}, "key": [70]}
and i am getting following while printing :
map[after:map[amount:811 id:123 status:Hi ] key:[70]]
Is there any way to print individual field like this??
amount::800
id:123
status:Hi
The code:
package main
import (
"encoding/json"
"fmt"
"io"
"io/ioutil"
"log"
"net/http"
"strings"
)
var (
PORT = ":8080"
)
func main() {
fmt.Println("In Main")
http.HandleFunc("/", changedData)
http.ListenAndServe(PORT, nil)
}
type Data struct {
Id int64 `json:"id"`
Amount float64 `json:"amount"`
Status string `json:"status"`
}
type mark map[string]interface{}
func changedData(w http.ResponseWriter, r *http.Request) {
fmt.Println("Coming From API")
reqBody, _ := ioutil.ReadAll(r.Body)
fmt.Println("Data coming from API ", string(reqBody))
digit := json.NewDecoder(strings.NewReader(string(reqBody)))
for digit.More() {
var result mark
err := digit.Decode(&result)
if err != nil {
if err != io.EOF {
log.Fatal(err)
}
break
}
fmt.Println("final_data ", result)
}
}

Answer 1

Decode to a Go type that matches the structure of the JSON document. You declared a type for the "after" field. Wrap that type with a struct to match the document.
func changedData(w http.ResponseWriter, r *http.Request) {
var v struct{ After Data }
err := json.NewDecoder(r.Body).Decode(&v)
if err != nil {
http.Error(w, "bad request", 400)
return
}
fmt.Printf("final_data: %#v", v.After)
}
Playground example.

Answer 2

I think you can define a struct type if you know the JSON file format or if the JSON format is predefined. As far as I know that mostly using interface{} is a way when you don't know the JSON format or there is no predefined format of the JSON. If you define a struct type  and use it while unmarshaling the JSON to struct, you can access the variables by typing like data.Id or data.Status.
Here's an example code:
package main
import (
"encoding/json"
"fmt"
)
type Data struct {
AfterData After `json:"after"`
Key []int `json:"key"`
}
type After struct {
Id int64 `json:"id"`
Amount float64 `json:"amount"`
Status string `json:"status"`
}
func main() {
j := []byte(`{"after": {"amount": 811,"id":123,"status":"Hi"}, "key": [70]}`)
var data *Data
err := json.Unmarshal(j, &data)
if err != nil {
fmt.Println(err.Error())
return
}
fmt.Println(data.AfterData)
fmt.Println(data.AfterData.Id)
fmt.Println(data.AfterData.Amount)
fmt.Println(data.AfterData.Status)
}
Output will be
{123 811 Hi}
123
811
Hi
Go Playground