Unmarshalling a single element in an array - json

Question

Is there a way to unmarshall JSON arrays into single objects in Go?
I have a json response from an endpoint:
{
"results": [
{
"key": "value"
}
]
}
I have a Go struct for the object inside the array:
type Object struct {
Key string `json:"key"`
}
...and a struct for the response object:
type Response struct {
Objects []Object `json:"results"`
}
results is an array of objects, but I know that the endpoint will only ever return an array with 1 object. Is there a way to unmarshall the data and avoid having reference the object by an index? I was hoping I could use something like json:"results[0]" as a field tag.
I'd prefer to be able to:
decoder.Decode(&response)
response.Object.Key
Rather than
decoder.Decode(&response)
response.Objects[0].Key

Answer 1

To does this you need to customize unmarshalling.
An way is create a ResponseCustom like:
//Response json (customized) that match with Unmarshaler interface
type ResponseCustom struct {
Object
}
func (r *ResponseCustom) UnmarshalJSON(b []byte) error{
rsp := &Response{}
err := json.Unmarshal(b, rsp)
if err != nil {
log.Fatalln("error:", err)
} else {
r.Object = rsp.Objects[0]
}
//
return err
}
So you can use ResponseCustom instead of you Response for get Object value.
Look:
func main() {
//
data := []byte(jsondata)
resp := &ResponseCustom{}
//
json.Unmarshal(data, resp)
//
fmt.Println("My Object.value is: " + resp.Object.Key)
}
The result is:
My Object.value is: value
In playground: https://play.golang.org/p/zo7wOSacA4w

Answer 2

Implement unmarshaler interface to convert array of object to object. Fetch the value for key and then unmarshal your json as
package main
import(
"fmt"
"encoding/json"
)
type Object struct {
Key string `json:"key"`
}
func (obj *Object) UnmarshalJSON(data []byte) error {
var v map[string]interface{}
if err := json.Unmarshal(data, &v); err != nil {
fmt.Printf("Error whilde decoding %v\n", err)
return err
}
obj.Key = v["results"].(interface{}).([]interface{})[0].(interface{}).(map[string]interface{})["key"].(interface{}).(string)
return nil
}
func main(){
var obj Object
data := []byte(`{
"results": [
{
"key": "value"
}
]
}`)
if err := json.Unmarshal(data, &obj); err!=nil{
fmt.Println(err)
}
fmt.Printf("%#v", obj)
}
Playground