Golang issue with accessing Nested JSON Array after Unmarshalling

I'm still in the learning process of Go but am hitting a wall when it comes to JSON response arrays. Whenever I try to access a nested element of the "objects" array, Go throws (type interface {} does not support indexing)

What is going wrong and how can I avoid making this mistake in the future?

package main    

import (
    	"encoding/json"
    	"fmt"
)    

func main() {
    	payload := []byte(`{"query": "QEACOR139GID","count": 1,"objects": [{"ITEM_ID": "QEACOR139GID","PROD_CLASS_ID": "BMXCPGRIPS","AVAILABLE": 19}]}`)
    	var result map[string]interface{}
    	if err := json.Unmarshal(payload, &result); err != nil {
    		panic(err)
    	}        

    	fmt.Println(result["objects"]["ITEM_ID"])    

}

http://play.golang.org/p/duW-meEABJ

edit: Fixed link

As the error says, interface variables do not support indexing. You will need to use a type assertion to convert to the underlying type.

When decoding into an interface{} variable, the JSON module represents arrays as []interface{} slices and dictionaries as map[string]interface{} maps.

Without error checking, you could dig down into this JSON with something like:

objects := result["objects"].([]interface{})
first := objects[0].(map[string]interface{})
fmt.Println(first["ITEM_ID"])

These type assertions will panic if the types do not match. You can use the two-return form, you can check for this error. For example:

objects, ok := result["objects"].([]interface{})
if !ok {
	// Handle error here
}

If the JSON follows a known format though, a better solution would be to decode into a structure. Given the data in your example, the following might do:

type Result struct {
	Query   string `json:"query"`
	Count   int    `json:"count"`
	Objects []struct {
		ItemId      string `json:"ITEM_ID"`
		ProdClassId string `json:"PROD_CLASS_ID"`
		Available   int    `json:"AVAILABLE"`
	} `json:"objects"`
}

If you decode into this type, you can access the item ID as result.Objects[0].ItemId.

For who those might looking for similar solution like me, https://github.com/Jeffail/gabs provides better solution.

I provide the example here.

package main

import (
	"encoding/json"
	"fmt"

	"github.com/Jeffail/gabs"
)

func main() {
	payload := []byte(`{
		"query": "QEACOR139GID",
		"count": 1,
		"objects": [{
			"ITEM_ID": "QEACOR139GID",
			"PROD_CLASS_ID": "BMXCPGRIPS",
			"AVAILABLE": 19, 
			"Messages": [ {
					"first": {
						"text":  "sth, 1st"
					}
				},
				{
						"second": {
						"text": "sth, 2nd"
					}
			  }
			]
		}]
	}`)

	fmt.Println("Use gabs:")
	jsonParsed, _ := gabs.ParseJSON(payload)
	data := jsonParsed.Path("objects").Data()
	fmt.Println("  Fetch Data: ")
	fmt.Println("    ", data)
	children, _ := jsonParsed.Path("objects").Children()
	fmt.Println("  Children Array from \"Objects\": ")
	for key, child := range children {
		fmt.Println("    ", key, ": ", child)
		children2, _ := child.Path("Messages").Children()
		fmt.Println("    Children Array from \"Messages\": ")
		for key2, child2 := range children2 {
			fmt.Println("      ", key2, ": ", child2)
		}
	}
}