Golang Unmarshal JSON - json

Question

My input JSON, has a list of different elements.
I have problems with the number of the first element of response.
Simplified example:
package main
import (
"fmt"
"log"
"encoding/json"
)
var j = ` {
"response": [
777, // problem here !!!
{
"id": 888,
"from_id": 999,
"to_id": 888,
"text": "hello..."
},
{
"id": 999,
"from_id": 888,
"to_id": 999,
"text": "goodbye..."
}
]
}`
type D struct {
Id int `json:"id"`
FromId int `json:"from_id"`
ToId int `json:"to_id"`
Text string `json:"text"`
}
type R struct {
Count int
Response []D `json:"response"`
}
func main() {
var data = new(R)
err := json.Unmarshal([]byte(j), &data)
if err != nil {
log.Fatal(err)
}
fmt.Println(data.Response)
}
Error on output. I do not understand where the error. Help me please.

Answer 1

1- Here is the working code,try it on The Go Playground:
package main
import (
"encoding/json"
"fmt"
)
func main() {
var d *R
err := json.Unmarshal([]byte(str), &d)
if err != nil {
panic(err)
}
var data R2
data.Count = int(d.Response[0].(float64))
for _, v := range d.Response[1:] {
bs, err := json.Marshal(v)
if err != nil {
panic(err)
}
var d1 *D
err = json.Unmarshal(bs, &d1)
if err != nil {
panic(err)
}
data.Response = append(data.Response, *d1)
}
fmt.Println(data)
}
type R struct {
Response []interface{} `json:"response"`
}
var str = ` {
"response": [
777,
{
"id": 888,
"from_id": 999,
"to_id": 888,
"text": "hello"
},
{
"id": 999,
"from_id": 888,
"to_id": 999,
"text": "goodbye"
}
]
}`
type D struct {
Id int `json:"id"`
FromId int `json:"from_id"`
ToId int `json:"to_id"`
Text string `json:"text"`
}
type R2 struct {
Count int
Response []D
}
output:
{777 [{888 999 888 hello} {999 888 999 goodbye}]}

Answer 2

Your response json is invalid. You use array instead of structure with fields specified in type D.

Answer 3

I have a different approach: https://play.golang.org/p/9Xnxk7tVxE
All you have to do is implement the Unmarshaler interface and add the logic inside the method UnmarshalJSON. It's a similar solution to the other one, but it's more portable since you don't have to do that in the main or external method and it's little more robust :)
package main
import (
"encoding/json"
"fmt"
"log"
)
var j = `{
"response": [
777,
{
"id": 888,
"from_id": 999,
"to_id": 888,
"text": "hello..."
},
{
"id": 999,
"from_id": 888,
"to_id": 999,
"text": "goodbye..."
}
]
}`
type D struct {
Id int `json:"id"`
FromId int `json:"from_id"`
ToId int `json:"to_id"`
Text string `json:"text"`
}
type R struct {
Count int
Response []D
}
func (r *R) UnmarshalJSON(data []byte) error {
var values struct {
Resp []interface{} `json:"response"`
}
if err := json.Unmarshal(data, &values); err != nil {
return err
}
if len(values.Resp) < 1 {
return fmt.Errorf("empty response %v", values.Resp)
}
count, isNumber := values.Resp[0].(float64)
if !isNumber {
return fmt.Errorf("first element has to be a number, we got '%T'", values.Resp[0])
}
r.Count = int(count)
for _, elem := range values.Resp[1:] {
de, err := json.Marshal(elem)
if err != nil {
return err
}
var d D
if err := json.Unmarshal(de, &d); err != nil {
return err
}
r.Response = append(r.Response, d)
}
return nil
}
func main() {
var data = new(R)
if err := json.Unmarshal([]byte(j), &data); err != nil {
log.Fatal(err)
}
fmt.Println(*data)
}