Traversing a Golang Map - go

Question

I initialized a variable named data like this:
var data interface{}
Then I unmarshalled raw json into.
err = json.Unmarshal(raw, &data)
I've run these two functions on it:
fmt.Println(reflect.TypeOf(data))
fmt.Println(data)
and those return this:
map[string]interface {}
map[tasks:[map[payload:map[key:36A6D454-FEEE-46EB-9D64-A85ABEABBCB7] code_name:image_resize]]]
and I need to access the "key". I have tried these approaches and a few more:
data["tasks"][0]["payload"]["key"]
data[0][0][0][0]
Those have all given me an error similar to this one:
./resize.go:44: invalid operation: data["tasks"] (index of type interface {})
Any advice on how to grab the "key" value out of this interface? Thanks in advance.

Answer 1

Since you already know your schema, the best way to do this is to unmarshal directly into structs you can use.
http://play.golang.org/p/aInZp8IZQA
package main
import (
"encoding/json"
"fmt"
)
type msg struct {
Tasks []task `json:"tasks"`
}
type task struct {
CodeName string `json:"code_name"`
Payload payload `json:"payload"`
}
type payload struct {
Key string `json:"key"`
}
func main() {
var data msg
raw := `{ "tasks": [ { "code_name": "image_resize", "payload": { "key": "36A6D454-FEEE-46EB-9D64-A85ABEABBCB7" } } ] }`
err := json.Unmarshal([]byte(raw), &data)
if err != nil {
fmt.Println(err)
return
}
fmt.Println(data.Tasks[0].Payload.Key)
}
If you insist on doing things the hard way using your original code, you need to do type assertions. I highly recommend avoiding this route when possible. It is not fun. Every step needs to be checked to ensure it matches the data structure you expect.
http://play.golang.org/p/fI5sqKV19J
if m, ok := data.(map[string]interface{}); ok {
if a, ok := m["tasks"].([]interface{}); ok && len(a) > 0 {
if e, ok := a[0].(map[string]interface{}); ok {
if p, ok := e["payload"].(map[string]interface{}); ok {
if k, ok := p["key"].(string); ok {
fmt.Println("The key is:", k)
}
}
}
}
}
In response to Goodwine's question: You can read further about how to marshal and unmarshal by reading the encoding/json godoc. I suggest starting here:
http://golang.org/pkg/encoding/json/#Marshal
http://golang.org/pkg/encoding/json/#Unmarshal

Answer 2

Another solution is to use 3rd-party package like https://github.com/Jeffail/gabs
With gabs, The example above by #stephen-weinberg can be written as:
raw := `{ "tasks": [ { "code_name": "image_resize", "payload": { "key": "36A6D454-FEEE-46EB-9D64-A85ABEABBCB7" } } ] }`
j, _ := gabs.ParseJSON([]byte(raw))
fmt.Println("The key is", j.S("tasks").Index(0).S("payload", "key").Data().(string))
other popular json handling packages I've stumbled upon are: https://github.com/bitly/go-simplejson and https://github.com/antonholmquist/jason.