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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