golang json unmarshal part of map[string]interface{}

Question

I have the following code to try to Unmarshal this json file, however the line json.Unmarshal([]byte(msg["restaurant"]), &restaurant) always gives an error. How can I make Unmarshal ignore the "restaurant" or pass only the "restaurant" data to the Unmarshal function?

Thanks!

{
  "restaurant": {
    "name": "Tickets",
    "owner": {
      "name": "Ferran"
    }
  }
}


    file, e := ioutil.ReadFile("./rest_read.json")
    if e != nil {
            fmt.Println("file error")
            os.Exit(1)
    }

    var data interface{}
    json.Unmarshal(file, &data)

    msg := data.(map[string]interface{})
    log.Println(msg)
    log.Println(msg["restaurant"])
    log.Println(reflect.TypeOf(msg["restaurant"]))

    var restaurant Restaurant
    json.Unmarshal([]byte(msg["restaurant"]), &restaurant)

    log.Println("RName: ", restaurant.Name)
    log.Println("Name: ", restaurant.Owner.Name)

Answer 1

I would propose to construct a proper model for your data. This will enable you to cleanly unmarshal your data into a Go struct.

package main

import (
    "encoding/json"
    "fmt"
)

type Restaurant struct {
    Restaurant RestaurantData `json:"restaurant"`
}

type RestaurantData struct {
    Name  string `json:"name"`
    Owner Owner  `json:"owner"`
}

type Owner struct {
    Name string `json:"name"`
}

func main() {
    data := `{"restaurant":{"name":"Tickets","owner":{"name":"Ferran"}}}`
    r := Restaurant{}
    json.Unmarshal([]byte(data), &r)

    fmt.Printf("%+v", r)
}

Answer 2

It is possible to do generic unmarshalling ala gson by decoding into an interface and then extracting a top level map from the result, e.g:

var msgMapTemplate interface{}
err := json.Unmarshal([]byte(t.ResponseBody), &msgMapTemplate)
t.AssertEqual(err, nil)
msgMap := msgMapTemplate.(map[string]interface{})

See "decoding arbitrary data" in http://blog.golang.org/json-and-go for more into.

Answer 3

Unmarshalling occurs recursively, so msg["restaurant"] is no longer a json string - it is another map[string]interface{}. If you want to unmarshall directly into a Restaurant object, you will have to provide a simple wrapper object with a Restaurant member and unmarshall into that.