Custom UnmarshalJSON: array object to map - json

Question

I have json files like this:
{
"Systems":[
{
"ID":74,
"Data1":0.1,
"Data2":4
},
{
"ID":50,
"Data1":31,
"Data2":3
}
],
"Error":false
}
I would like to unmarshal in Go to something like this (note map):
type Info struct {
Systems map[int]System `json:"Systems"` // key should be ID json field
Error bool `json:"Error"`
}
type System struct {
Data1 float32 `json:"Data1"`
Data2 int `json:"Data2"`
}
Here is my (wrong) code:
package main
import (
"encoding/json"
"fmt"
)
type Info struct {
Systems map[int]System `json:"Systems"` // key should be ID json field
Error bool `json:"Error"`
}
type System struct {
ID int `json:"ID"`
Data1 float32 `json:"Data"`
Data2 int `json:"Data2"`
}
func main() {
file := "{\"Systems\":[{\"ID\":74,\"Data1\":0.1,\"Data2\":4},{\"ID\":50,\"Data1\":31,\"Data2\":3}],\"Error\":true}"
info := Info{}
bytes := []byte(file)
err := json.Unmarshal(bytes, &info)
if err != nil {
fmt.Printf("=> %v\n", err)
}
fmt.Printf("INFO: %+v\n", info)
}
func (d *Info) UnmarshalJSON(buf []byte) error {
var tmp interface{}
if err := json.Unmarshal(buf, &tmp); err != nil {
return err
}
d.Error = tmp.(map[string]interface{})["Error"].(bool)
d.Systems = make(map[int]System)
for _, v := range tmp.(map[string]interface{})["Systems"].([]interface{}) {
d.Systems[v.(map[string]interface{})["ID"].(int)] = v.(map[string]interface{}).(System)
}
return nil
}
https://play.golang.org/p/L_Gx-f9ycjW

Answer 1

You can try this
package main
import (
"encoding/json"
"fmt"
)
func main() {
var str = `{
"Systems":[
{
"ID":74,
"Data1":0.1,
"Data2":4
},
{
"ID":50,
"Data1":31,
"Data2":3
}
],
"Error":false
}`
var t Info
err := json.Unmarshal([]byte(str), &t)
if err != nil {
panic(err)
}
bts, err := json.MarshalIndent(t, "", " ")
if err != nil {
panic(err)
}
fmt.Println(string(bts))
}
type Info struct {
Systems map[int]System `json:"Systems"`
Error bool `json:"Error"`
}
type System struct {
ID int `json:"ID,omitempty"`
Data1 float32 `json:"Data1"`
Data2 int `json:"Data2"`
}
func (info *Info) UnmarshalJSON(data []byte) error {
var t struct {
Systems []System `json:"Systems"`
Error bool `json:"Error"`
}
err := json.Unmarshal(data, &t)
if err != nil {
return err
}
info.Systems = make(map[int]System, 0)
for _, v := range t.Systems {
info.Systems[v.ID] = v
}
return nil
}
https://play.golang.org/p/qB3vF08cmW8
Output:
{
"Systems": {
"50": {
"ID": 50,
"Data1": 31,
"Data2": 3
},
"74": {
"ID": 74,
"Data1": 0.1,
"Data2": 4
}
},
"Error": false
}

Answer 2

You can't decode json directly in the struct you proposed because it doesn't match the json structure.
What you can do is decoding the json into this:
type Info struct {
Systems []*System `json:"Systems"` // array here
Error bool `json:"Error"`
Index map[int]*System // not json mapped
}
type System struct {
ID int `json:"ID"`
Data1 float32 `json:"Data1"`
Data2 int `json:"Data2"`
}
and populate the Index field in postprocessing with something like this:
var info Info
json.Unmarshal(dataIn, &info)
info.Index = map[int]*System{} // initialize an empty map
for _, s := range info.Systems {
info.Index[s.ID] = s
}
fmt.Println(info.Index[50].Data1)
you can find a full example here https://play.golang.org/p/B8O6nfI258-