CODEWITHSUNDEEP
jsongo
The user with no hat
The user with no hat

Reputation: 10846

Is it possible to partially decode and update JSON? (go)

I need to decode and update only a specific value of a json object. The issue is that I don't know the full structure of the object. The encoding/json package "ignores"/truncates the fields not provided in the struct so on encoding these fields are lost. I'm wondering if it's possible to unmarshal only the structure I know, update it and then marshal it without to truncate/remove the unknown structure/information.

Upvotes: 3

Views: 4824

Answers (2)

Shinichi TAMURA
Shinichi TAMURA

Reputation: 150

I know this is quite old question, but I learned combination of usual struct and json.RawMessage will do the job in the situation. Let me share.

The point is: hold entire data into raw field, and use that for encoding/decoding. Other fields can be derived from there.

package main

import (
    "encoding/json"
    "log"
)

type Color struct {
    Space string
    raw   map[string]json.RawMessage
}

func (c *Color) UnmarshalJSON(bytes []byte) error {
    if err := json.Unmarshal(bytes, &c.raw); err != nil {
        return err
    }
    if space, ok := c.raw["Space"]; ok {
        if err := json.Unmarshal(space, &c.Space); err != nil {
            return err
        }
    }
    return nil
}

func (c *Color) MarshalJSON() ([]byte, error) {
    bytes, err := json.Marshal(c.Space)
    if err != nil {
        return nil, err
    }
    c.raw["Space"] = json.RawMessage(bytes)
    return json.Marshal(c.raw)
}

func main() {
    before := []byte(`{"Space": "YCbCr", "Point": {"Y": 255, "Cb": 0, "Cr": -10}}`)
    log.Println("before: ", string(before))

    // decode
    color := new(Color)
    err := json.Unmarshal(before, color)
    if err != nil {
        log.Fatal(err)
    }

    // modify fields of interest
    color.Space = "RGB"

    // encode
    after, err := json.Marshal(color)
    if err != nil {
        log.Fatal(err)
    }
    log.Println("after:  ", string(after))
}

The output should be like this:

2020/09/03 01:11:33 before:  {"Space": "YCbCr", "Point": {"Y": 255, "Cb": 0, "Cr": -10}}
2020/09/03 01:11:33 after:   {"Point":{"Y":255,"Cb":0,"Cr":-10},"Space":"RGB"}

NB: this doesn't preserve key order or indentations.

Upvotes: 3

The user with no hat
The user with no hat

Reputation: 10846

It seems it's possible. 

package main

import (
    "encoding/json"
    "fmt"
    "log"
)

func main() {
    type Color struct {
        Space string
        Point json.RawMessage // delay parsing until we know the color space
    }
    type RGB struct {
        R uint8
        G uint8
        B uint8
    }
    type YCbCr struct {
        Y  uint8
        Cb int8
        Cr int8
    }

    var j = []byte(`
        {"Space": "YCbCr", "Point": {"Y": 255, "Cb": 0, "Cr": -10}}`)
    var colors Color
    err := json.Unmarshal(j, &colors)
    if err != nil {
        log.Fatalln("error:", err)
    }
    colors.Space = "no-space"

    b, err := json.Marshal(&colors)
    if err != nil {
        panic(err)
    }
    fmt.Printf("b is now %s", b)
    return

}

Upvotes: 1

Related Questions