Traversing a Golang Map

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.

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

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.