Golang issue with accessing Nested JSON Array after Unmarshalling - json

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.

Answer 2

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)
}
}
}