How to parse an inner field in a nested JSON object - json

Question

I have a JSON object similar to this one:
{
"name": "Cain",
"parents": {
"mother" : "Eve",
"father" : "Adam"
}
}
Now I want to parse "name" and "mother" into this struct:
struct {
Name String
Mother String `json:"???"`
}
I want to specify the JSON field name with the json:... struct tag, however I don't know what to use as tag, because it is not the top object I am interested in. I found nothing about this in the encoding/json package docs nor in the popular blog post JSON and Go. I also tested mother, parents/mother and parents.mother.

Answer 1

You could use structs so long as your incoming data isn't too dynamic.
http://play.golang.org/p/bUZ8l6WgvL
package main
import (
"fmt"
"encoding/json"
)
type User struct {
Name string
Parents struct {
Mother string
Father string
}
}
func main() {
encoded := `{
"name": "Cain",
"parents": {
"mother": "Eve",
"father": "Adam"
}
}`
// Decode the json object
u := &User{}
err := json.Unmarshal([]byte(encoded), &u)
if err != nil {
panic(err)
}
// Print out mother and father
fmt.Printf("Mother: %s\n", u.Parents.Mother)
fmt.Printf("Father: %s\n", u.Parents.Father)
}

Answer 2

Unfortunately, unlike encoding/xml, the json package doesn't provide a way to access nested values. You'll want to either create a separate Parents struct or assign the type to be map[string]string. For example:
type Person struct {
Name string
Parents map[string]string
}
You could then provide a getter for mother as so:
func (p *Person) Mother() string {
return p.Parents["mother"]
}
This may or may not play into your current codebase and if refactoring the Mother field to a method call is not on the menu, then you may want to create a separate method for decoding and conforming to your current struct.

Answer 3

Here's some code I baked up real quick in the Go Playground
http://play.golang.org/p/PiWwpUbBqt
package main
import (
"fmt"
"encoding/json"
)
func main() {
encoded := `{
"name": "Cain",
"parents": {
"mother": "Eve"
"father": "Adam"
}
}`
// Decode the json object
var j map[string]interface{}
err := json.Unmarshal([]byte(encoded), &j)
if err != nil {
panic(err)
}
// pull out the parents object
parents := j["parents"].(map[string]interface{})
// Print out mother and father
fmt.Printf("Mother: %s\n", parents["mother"].(string))
fmt.Printf("Father: %s\n", parents["father"].(string))
}
There might be a better way. I'm looking forward to seeing the other answers. :-)

Answer 4

More recently, gjson supports selection of nested JSON properties.
name := gjson.Get(json, "name")
mother := gjson.Get(json, "parents.mother")

Answer 5

How about using an intermediary struct as the one suggested above for parsing, and then putting the relevant values in your "real" struct?
import (
"fmt"
"encoding/json"
)
type MyObject struct{
Name string
Mother string
}
type MyParseObj struct{
Name string
Parents struct {
Mother string
Father string
}
}
func main() {
encoded := `{
"name": "Cain",
"parents": {
"mother": "Eve",
"father": "Adam"
}
}`
pj := &MyParseObj{}
if err := json.Unmarshal([]byte(encoded), pj); err != nil {
return
}
final := &MyObject{Name: pj.Name, Mother: pj.Parents.Mother}
fmt.Println(final)
}