Golang issue with accessing Nested JSON Array after Unmarshalling

Question

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

Answer 1

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.