identify the type of an object dynamically while reading json data

Question

Quite new to go, sorry if this question sounds obvious.

I would like to use reflection in order to identify the type of an object while reading a json file.

The use case (please see the code below) is the following: I have two structs BoyGift and GirlGift that contain different fields. I have also a Boolean indicator IsBoy that is true if the recipient of the giftis a boy, false otherwise.

The type that encapsulates this behavior is the type Gift:

//Gift type
type Gift struct {
    IsBoy   bool     `json:"isBoy"`
    Payload ??? `json:"payload"`
}

that holds the data. How can I define that type in order the json unmarshal to convert dynamically to the correct type? The "json schema" in this case defines that a Gift should be either a BoyGift or a GirlGift. Is it possible to do this via reflection? How?

Doing unmarshal twice would be great, if the Boolean info is known

package main

import (
    "encoding/json"
    "fmt"
)

//BoyGift type
type BoyGift struct {
    Cars  uint32 `json:"cars"`
    Balls uint32 `json:"balls"`
}

//GirlGift type
type GirlGift struct {
    Dolls uint32 `json:"dolls"`
    Lego  uint32 `json:"lego"`
}

//Gift type
type Gift struct {
    IsBoy   bool     `json:"isBoy"`
    Payload GirlGift `json:"payload"`
}

func main() {

    b := []byte(`{
        "isBoy": true,
        "payload": {
          "cars": 1,
          "balls": 2
        }
      }`)

    var g Gift
    err := json.Unmarshal(b, &g)

    if err != nil {
        fmt.Println(err)
        return
    }

    fmt.Println(g)
}

It is not possible to create dynamic type for a struct field. If you are not sure you can use interface and then assign it to the structs given. — Himanshu
– Himanshu, Commented Jul 19, 2018 at 12:20

MichalG · Accepted Answer · 2018-07-19 12:32:09Z

You should use json.RawMessage to dynamically unmarshal your data.

You can define Gift's Payliad as json.RawMessage and then delay unmarshaling until you know the value of IsBoy. Below you can find a basic example of how to do it.

package main

import (
    "encoding/json"
    "fmt"
)

//BoyGift type
type BoyGift struct {
    Cars  uint32 `json:"cars"`
    Balls uint32 `json:"balls"`
}

//GirlGift type
type GirlGift struct {
    Dolls uint32 `json:"dolls"`
    Lego  uint32 `json:"lego"`
}

//Gift type
type Gift struct {
    IsBoy   bool            `json:"isBoy"`
    Payload json.RawMessage `json:"payload"`
}

func main() {

    b1 := []byte(`{
        "isBoy": true,
        "payload": {
          "cars": 1,
          "balls": 2
        }
      }`)

    b2 := []byte(`{
        "isBoy": false,
        "payload": {
          "dolls": 3,
          "lego": 4
        }
      }`)

    for _, b := range [][]byte{b1, b2} {
        var g Gift

        err := json.Unmarshal(b, &g)
        if err != nil {
            fmt.Println(err)
            return
        }

        if g.IsBoy {
            var boyGift BoyGift
            err := json.Unmarshal(g.Payload, &boyGift)
            if err != nil {
                fmt.Println(err)
                return
            }
            fmt.Println(boyGift)

        } else {
            var girlGift GirlGift
            err := json.Unmarshal(g.Payload, &girlGift)
            if err != nil {
                fmt.Println(err)
                return
            }
            fmt.Println(girlGift)

        }
    }

}

If you want to use Payload as an interface{} (which can be BoyGift or GirlGift), you could create additional helper struct for unmarshalling. Check extended example in Go Playground: https://play.golang.org/p/q1Hn45bgjsc

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